A lot of the advertising, merchandise, and fanart surrounding Sluggy Freelance involves Oasis. Enough so that someone who starts reading the series from the beginning will probably guess something's up when she "dies" at the end of her introductory story in 1999, although it's lampshaded even then.
El Goonish Shive: Ellen exists, and she doesn't stay a villain; Grace can shapeshift, and Tedd doesn't need glasses.
Still, the comic gets an honorable mention for (initially at least) having a cast page split between "spoilers" and "not spoilers".
Though Angel Moxie was good about this during its run, the website is not coy about such things now that the series is over and has rerun several times. The girls are shown in the powered-up forms they don't get until almost the end of the series, which also blows the revelation that all three girls are Legendary Heroes and not just the Magical Girl. The site synopsis is also just one giant spoiler of every plot point in the series.
Dan and Mab's Furry Adventures: Dan's mother being a succubus, and himself being a cubi whose powers had not yet awakened, was a major plot twist when it happened, but it's so ingrained with everything that happens later that the fans make no attempt to conceal it.
One especially egregious example is Family Man (Dylan Meconis) by Dylan Meconis. It is almost ubiquitously hailed as a superb werewolf story ... and has been since before a single werewolf appeared in the story, much less The Reveal. (The reason for this is obvious: the story is a semi-prequel to Bite Me!, in which Luther was a werewolf.)
Parodied in Ansem Retort. Someone yells at Marluxia for ruining the fact that Qui-Gon Jinn dies in The Phantom Menace. That someone? Darth Maul.
Kevin & Kell: Lindesfarne and Danielle are both from the human world.
The Order of the Stick books come with informative chapter introductions... that tell you bits of what is going to happen in the next chapter. And later chapters. And sometimes in later books. The assumption seems to be that nobody will be reading the books without having first read every strip online.
Though the first book does include a suggestion that you read the strips before the extra text.
Not much of a spoiler, but someone who has not yet finished reading the strip may wonder why Haley is holding a sign saying "I.O.Me: One big-ass diamond" on the cast page.
This is especially true of the prequel books. The first book chronologically spoils almost every background twist for the first three mainline books and holds the payoff for a Brick Joke in the chronologically second book (which is still a prequel but released years earlier); the expectation seems to be that they'll be read in release order.
Paradigm Shift: Every page has badge graphics for each act of the series. The badge for "Flight" makes it pretty clear what Kate's story is.
The Secret Blueprints that were released at the same time as the first issue (back when it was released as comic books first) spoiled not only Agatha's identity, but also Adam and Lilith's.
After around a decade of the webcomic running, the Foglios largely stopped bothering to hide the fact that Agatha was a Heterodyne, which was revealed in Volume III of the webcomic or around issues #9 or #10 of the original print comic (though as noted above the Secret Blueprints exposed it earlier). Kaja tried to soften the spoiler with the title of the first prose novel, Agatha H. and the Airship City, shortening her last name to "H." so at least readers can tell it's not really "Clay".
The "Complete List of Absolutely Everybody!" — as well as the later character sheets for Vols. VII, VIII, & IX — give spoilers for Agatha being a Heterodyne, Lucrezia Mongfish being the Other, Adam & Lilith being Punch & Judy, and Dr. Beetle knowing of Barry's return.
The seventh Electric Wonderland comic dramatically revealed Lululu's mermaid tail. After Peter Paltridge wrote some character bios in June 2010, newcomers who clicked the "newbie? go here!" button on the Platypus Comix home page could find out about her tail beforehand. The bios also spoil the fact that Natasha Wing, the seemingly random police girl who appeared at the end of the sixth comic, is actually friends with protagonist Trawn.
A variation: Chapter 6 of Magellan carried a mild spoiler for Crossoverkill: Hoodoo not only survives unscathed, but is pregnant with Captain Perfect's child. Word of God says this is due to Schedule Slip on Crossoverkill - it was meant to be completed long before the Magellan story started.
In the first issue of The Demon Mages, the actress Ari passed off as a "human, reportedly". Not only is she referred and portrayed as a Gorgon everywhere else she appears (such as the Demon Mages' Character Blog and on her creators DeviantArt page), the second issue simply outs her as one without much build-up.
As L's Empire's profile page shows all of the authors, it's nigh impossible to not know about Dark Star and the fact that he becomes an author.
Darths & Droids tries to avert this by having a separate cast page for each episode.
Zebra Girl: The cast page is not even trying to hide many key events of the story. Particularly the way Sandra eventually succumbs to her demonic urges and becomes a ruthless and cruel demon.
Sleepless Domain: The series is initially presented as an episodic Magical Girl Warrior story starring the five girls of Team Alchemical... but a massive First-Episode Twist early on shifts the focus to the team's supporting member Undine and reveals the other four girls to be Decoy Protagonists. Most promotional materials, to their credit, only vaguely allude to the specifics of the twist in question, but very little attempt is ever made to conceal the series' true main character — to the extent that even the very first volume features Undine front and center on the cover and gives away her protagonist status in the blurb.
TwoKinds: Flora (a Tiger Keidrian) is pregnant with Human Trace's child - something thought to be impossible, in-universe. Secondary character Raine has been a Human/Keidrian hybrid all along.
The fact that It Hurts!! suddenly shifts from a Slice of Life setting to a supernatural, sci-fi post-apocalypse is not kept secret by any aspect of the site. The banner image, for example, is occasionally a headshot of Zombie!Allison.
Questionable Content: Faye is a recovering alcoholic who may or may not have tried to kill herself (even she's not sure). Claire is a trans woman. Whether or not she's had reassignment surgery is intentionally never brought up.
Stand Still, Stay Silent: The main character that dies during the first story arc is very obviously absent from illustrations made for the second arc that are supposed to include the entire main cast.
While it updated regularly, Schlock Mercenary had a general fansite/fan group policy of 'Wait 48-72 hours to mention anything (including audience reaction, in some cases), but with the foreshadowing and callbacks, you could really find a Late-Arrival Spoiler in something mentioned from 15 years ago. Including Laz'R'Us tech, the importance of the core generator, the fact that there are two Kevyn's, Ennesby being able to jump between and control different computer systems...
The "Awful Hospital" button on the Bogleech main page would take the reader to the most recent update, which could spoil a lot for a new reader, even out-of-context. Thankfully, the button was eventually replaced with two separate buttons that lead to the first page and newest page respectively.
Sarah, I'm Sorry: Welcome to Our Galaxy is a spinoff of Mom, I'm Sorry that thoroughly spoils the ending of the source material. Unfortunately for English readers, Sarah, I'm Sorry was fully translated months before the conclusion of Mom, I'm Sorry was translated.
NEXT!!! Sound of the Future: Shine's inability to sing was kept hidden in all prerelease material and was only revealed in the comic itself in chapter 2. After The Reveal happened in the story it was included in the official series description, so any new readers will know right from the beginning.
Penny Arcade details this very phenomenon, regarding movies. Specifically King Kong (2005) and The Passion of the Christ. As Gabe puts it, there's a statute of limitations on this kind of thing.