Gather round people, cos I've got a good one:
I had bought a copy of Diamond on it's release date in Australia. Normal cartridge, I never dropped it, no problem.
Anyway, seeing as I had never played a 4th gen game before (duh), I caught a Shinx. This Shinx, was a Normal/Electric type. Thinking it was part of the show (I later checked online guides and found out it's uniqueness) I pressed on. This Shinx had a problem though. No matter what Pokemon I fought, it's electric moves would always be 'Not very effective!'. This applied for fighting ground types, and water types. But the dodgy Shinx wasn't all. I also caught a Haunter, that evolved into a Gengar at level 26, and a Psyduck that was a pure Psychic type, which later evolved into a normal Golduck.
I have absolutely NO idea what was wrong with the cartridge, but there were some messed up things about it (some trainer spoke gibberish). It's been a year since I sold it, and I've been kicking myself ever since.
fake cartridge =/= glitch