I've noticed in a few raids that sometimes my dispel magic spell will cast, and remove the mana from my pool, but will not remove any debuffs. This does happen when I use the spell defensively (on friendly targets.) I tracked back in my combat log, and it said something about the spell failing.

Is this a bug that others have experienced?

If not, could it possibly be some discrepancy between the server/client? Is my horrendous video lag and latency catching up with me? Will Batman find the Joker's lair in time to save Robin? Stay tuned.

Magic/poison/disease effects have a chance to resist being dispelled. See the improved Rogue talent Vile Poisons that improves the chance the poison will not be dispelled.

dark plague in scholo is a particular nasty one, I remember casting cleanse roughly 7 times on one occasion (and no, the debuff wasnt stacked)
makes me wonder if +spell hit gear actually improves the chance of succesfully cleansing
probably not since cleanse is allied targets only, thus guaranteed to hit, but it would be nice if it did work so you could at least gear up for some non-existent encounter with a hard-to-cleanse debuff that needs to get cleansed very fast :)