Comics have a consistent release schedule that leaves a lot of room for mistakes, and yet a surprisingly low number of errors make it into print.
It's easy to assume syndicated comics would be full of errors since they're (generally) released daily, but errors that occur are usually the publisher's fault and not the creator's, even though it's the creators who feel the backlash in the end.
Garfield's Jim Davis felt this backlash back in 2010 when a strip he'd drawn nearly a year earlier was released on Veteran's Day and taken out of context as an insult against veterans.
In the world of comic books screw-ups are usually a result of an artist or writer's poor judgment, since errors are generally caught by editors before the issue is printed.
Stan Lee was working as editor when he allowed Marvel's Tales Of Suspense #92 to be printed with a terrible joke told by Captain America, a guy not generally known for his clever quips.
The joke was seen as "one of the funniest lines of dialogue in cartooning history", and later reprints feature a "corrected" version of the joke.