This microphone smells funny. You find that old boot? Plant it. You find a rusty, old pocket watch? Plant it. Find a copy of Terminator 2 on laserdisk? Plant it. You find a pair of socks? Plant it. You find a Milli Vanilli cassette tape? Plant it.
Upon equipping the Fire Bellows and the War Machete:
"We could have ourselves a mighty fine cookout with those items. Too bad that's not what we're doing."
Or the Pike and Bellows:
"Cook em and stick em... or the other way around. Makes no difference."
Or the Bellows and Galleon Mortar:
"Well, if you can't burn it, you can always blow it to Mother-on-High, right?"
If you manage to trick Gershel (the big scumbag) into rolling off the edge:
"Shame old Gershel can't float like when he was a young fella."
Anytime that a particularly hard boss is defeated by... falling off the playfield. This tends to happen a lot with Scumbags and Lunkheads.
Becomes a hair-pulling situation as the final enemy of the Shield Proving Ground forces you to do this.
Killing a boss with the explosion-on-death effect from Hearty Punch is pretty funny, too, especially if you weren't expecting it/forgot you had it on. Queen Anne the Anklegator is particularly vulnerable to this.
The Kid lands at any given area... flat on his face.
If you are about to leave the Bastion and opt to return to the Bastion instead of going anywhere, the Kid has already taken a leap up to the sky... so he just falls down and lands flat on his face.
Especially at the beginning of the Sundown Path. The camera focuses on a bale of hay for a good few seconds, then zooms out, and the Kid faceplants into the level a little to the left. In contrast, he DOES crash into various, far from ideal, objects like weapon turrets and crates.
Showing the gel canister (a special item on the Steam version that summons a mini-turret from Portal) to Zulf gets you this reaction.
"'You show me whoever made this,' Zulf says, 'and I'll show you a scientific genius.'"
Rucks' announcer cameo in Dota 2 has plenty of these.
Especially funny if you consider your drink combinations as something the Kid is drinking every time he goes out. With how strong he gets and how many weapons he ends up handling, he ends up coming off as a completely wasted daredevil.
You enter Zia's Who Knows Where through her stockpot. If you hit the Outhouse a few times, Rucks will mention that her cooking is "getting to him". Which makes the image of Zulf in the Evacuation Ending actually pretty funny: everyone leaves Zia to her adventures and him to the kitchen.