Seriously, no one mentioned the time the first half of an episode of Ruroni Kenshin aired instead of Hamtaro?
I thought it was the other way around ^^;;
I also remember recently, a new Gumball episode aired in an arbitrary weekday morning slot, despite being advertised to air in its regular primetime slot. The bumper even said, "Tuesday at 8pm - All New Gumball!" many times during it.
A Scaredy Squirrel episode also had this - originally it was supposed to air Wednesday night, though the show only aired on weekday mornings and weekend afternoons. It aired one weekend afternoon, without the "NEW EPISODE" bug, and my DVR caught it, but when it aired again one morning after that, the "NEW EPISODE" bug was on that rerun.
A few times, the [adult swim] bug came up from the corner instead of the Cartoon Network bug. The most famous example was during the One Piece episode "No Way Out" I think back in 2007, but it's happened before at one point with something weirder...
One morning, I saw the Foster's movie "House of Bloo's". I think this was back in '05. The movie started from the second act, which was replayed and continued as normal. But from the second act onward, the [adult swim] bug was seen once again.
One thing I remember that infuriated fans was the Naruto episode "The Third Hokage Forever." I never saw this error, but from what I heard, during a scene where Kakashi said, "I've been here since d-" (dawn) that line was repeated, followed by a Cartoon Network fence, then the credits were cut to 10 minutes early, messing up the tape and causing One Piece, MAR, and The Prince of Tennis all to start early - even [adult swim] started early.