I'm not sure about the Discworld example. The trope description is pretty clearly "Alice says 'Bob had better not do X' and Bob says 'Great idea, I'll do X now'", and this isn't that.
Also "There are not 2,300,009 invisible bloodsucking ghosts" isn't actually a warning anyway, it's saying there's nothing to be warned about. But the soldiers do, as Rincewind intended, take it as a warning, so if it has anything to do with the trope, it's an ... invoked inversion? But I think it just isn't really related at all.
I'm not sure about the Discworld example. The trope description is pretty clearly "Alice says 'Bob had better not do X' and Bob says 'Great idea, I'll do X now'", and this isn't that.
Also "There are not 2,300,009 invisible bloodsucking ghosts" isn't actually a warning anyway, it's saying there's nothing to be warned about. But the soldiers do, as Rincewind intended, take it as a warning, so if it has anything to do with the trope, it's an ... invoked inversion? But I think it just isn't really related at all.
Edited by DaibhidC