Back in the bleak days of pre-BC raiding, mobs had a very constricting debuff limit. No more than 16 dots and debuffs combined could be present on a mob at any one time, which made the use of many of our dots a problem on raids.
The Burning Crusade brought about two changes that made the debuff limit practically a non-issue. For one, the raids for BC instances aren't for more than 25 people. Secondly, the debuff limit itself was increased to 40.
Even though it is no longer a problem, I'll explain how the 16 debuff limit used to affect raiding;
Imagine being in a raid, fighting a boss. You cast corruption, immolate and curse of doom on the target. Seconds later, your spells are no longer visible in the list, and have been replaced by abilities such as rend, thundersclap, taunt, demoralizing shout etc. Your three spells have been ‘pushed off’ the debuff list. They aren't just missing from the debuff list, they are completely replaced by other abilities. Its as if they were dispelled.

Back in those days, if you wanted any debuff to stay on the target, you would have to recast it frequently to ensure it stayed on there. This is obviously a waste of mana when done with DoTs, so most locks had to forego using most of their DoTs. Many raiding guilds had rules in places for which debuffs may be cast and which may not. Some used to ban talents such as deep wounds and improved shadowbolt, as they cannot be prevented from going off and could push a needed debuff off.
Sweet times, huh?

The above is now of little concern to the raiding lock, but I still thought you should be aware of the way that used to work. And it's not entirely impossible for this to happen with the current system, as the possibility of reaching 40 debuffs on a 25 man raid is definitely still there.