Some people here have glitches and bugs mixed.
A glitch is random.
A bug is a glitch you can replicate.
On-topic: The best glitch I have ever experienced was in Dungeon Siege: Throne of Agony. I somehow gained experience for 32 levels for me(68 to 100) and enough to get all followers to 100.(lowest followers were in the early 40s) It's not that leveling is difficult in ToA but it made me want to fully understand the game, and for some time I might have been the only person that fully understood the game mechanics.
The best bug in any game is health drain stacking. Yes, it's nice you can poison a target with 4 damage per second but at least put a limit on how many of the damage effects can stack. If you don't, well, Throne of Agony has a great example of what happens. A level 40(10 levels under the average, still too high) wizard with no stats or skills distributed can kill the final boss in a few seconds(depends on bow used and criticals) by using a bow and the fire enchant which does 1% health damage every second for 4 seconds and every hit would add another 1% damage per second.
You could call this a game mechanic but it's just lazy programming. It's not difficult to have a script checking if the health drain is higher than X per second for anything not related to a poison(spell) and have the new health drain remove the old one.