If you understand the bug in question, and you are using it to win in a competition, then it is cheating. You're taking advantage of something for your own benefit and you are using it to get ahead of other players, giving you an unfair advantage against competitors who may not know the glitch in question. But a big question is how can you tell whether they were doing it on purpose?
This reminds me of the controversy behind the defeat of Arthas, the Lich King in WoW: Wrath of the Lich King, as written about in [user]John Funk[/user]'s column A View From The Road [http://www.escapistmagazine.com/articles/view/columns/view-from-the-road], under the article Fail to the King [http://www.escapistmagazine.com/articles/view/columns/view-from-the-road/7139-A-View-From-The-Road-Fail-to-the-King]; the first team to defeat the Lich King in 25 Man Mode, Ensidia, managed to do so by exploiting a bug in the way the arena would be destroyed over time, and by doing so, managed to defeat Arthas. Blizzard found out about this, though, and fixed the bug, stripped Ensidia of their winnings from that battle and suspended them all. While this would seem fairly normal, Ensidia got pissed, saying in their member's [http://ensidia.com/muqq/blog/4078/] blogs [http://www.ensidia.com/Jinxarn/blog/4090/] that the strategy they were using to exploit the bug was a strategy that their members use as standard, it's just that when they used this strategy in the winning battle they noticed the effect that it had on the environment and took advantage of it then.
The only people who could've seen Ensidia take advantage of the bug is Ensidia, and they have a vested interest to say that they didn't mean to exploit the bug, due to their suspensions for it and them being stripped of their treasure as a result of it, so they can't be fully trusted as a source. So who's to say whether or not they meant to do it?
As for Single Player, this is completely different. Helping you beat a computer with glitches and errors isn't cheating per se, as there isn't anyone to have an unfair advantage against. This is the main reason why exploiting bugs in video games to finish quickly has started to become less of a disgraceful activity and more of an artform [http://www.escapistmagazine.com/articles/view/issues/issue_207/6192-Zen-and-the-Art-of-Speedrunning] over time.