As others have said, it's a debuff limit on mobs. Once you get near that limit, skills will occasionally not show up at all even if you have it set to show only your own. What's more is that while they'll be active and do damage even when not showing up for you, it is my understanding that they can get to the point where they simply won't activate on the enemy at all if they reach the hard limit, doing no damage or debuffs. This includes any skill that activates on enemy that doesn't just do instant damage. Some, (but not all), skills that it's notably annoying that this can affect are Virus, Eye for an Eye, Trick Attack, and the one that is the most annoying for me in particular, Wildfire.....

For those of you who don't play MCH, Wildfire is by far our highest DPS skill if used properly, (dealing 1/5 of the damage of our 15s burst rotation on it's own), while being tied to a 90s cooldown after casting it. It can also be completely disabled if there are too many effects on the target, resulting in a fairly massive drop in MCH DPS if it happens multiple times in one fight, (which it almost assuredly will if it happens once). This is on the class that is tied with Bard as having the lowest base DPS to begin with, and that relies on that attack to reach that point. It's not as big of a deal if a single DoT is knocked off for a SMN or other classes, because while it does add up, they can simply recast that skill when something else wears off whenever they want. With MCH, this skill is tied to a 90s cooldown which means that if it doesn't apply but the skill is used, we simply lose out on that skill entirely for a minute and a half, (actually longer if you want to make the most out of it when you do use it)......

The status limit on enemies is a system that desperately needs to be reworked. Particularly as they add more and more skills that inflict some sort of status effect or DoT to classes, because it will get to a point where skills are constantly being prevented from activating on enemies in 24 player content simply because half the group is using all their skills. For point of reference, what I keep hearing is that that the visual DoT limit on enemies is around 40, and I think I've heard that the hard limit is around 50, (though it could have been less or more on the hard limit). Let's say in the extreme case, you go into a 24 player raid and the RNG hates your group, so you get 5 SMNs and 2 SCHs per party. JUST from them using Bio, Bio 2, Miasma, and Miasma 2, and JUST from the DoTs and not counting status effects from these skills, that would be 84 DoTs, plus another 6 Aero's from the SCHs, bringing us up to 90 potential DoTs active on a target at once. If the hard limit is 50, that means that 40 DoTs are completely lost even before accounting for skills like Virus and Eye for an Eye taking up slots, or even Shadow Flare, not to mention whatever tank skills are used that inflict debuffs. That would be all four DoTs from 10 members of your raid group that would be doing nothing what so ever, which means that potentially the entire DPS from two parties would be stuck without most of their skills able to do any damage. Again, this is in the most extreme case obviously, but it is a very real concern since so many classes have at least one or two DoTs, and a handful of other skills that inflict debuffs of some sort. Going back to MCH, I could potentially have 4 active skills on an enemy during the start of my burst that would take up a slot, even if they're immune to silence, and my theoretical situation was listing just 4 slots being taken by each member of the raid group minus the tanks. In most cases, all the slots might not be filled up CONSTANTLY by DoTs and debuffs, but just being filled up most of the time can result in a massive amount of damage and skills lost across the raid group....

I can understand why there are limits on this sort of thing, because it is necessary to limit just how much data is being sent back and forth from the server during large scale fights in order to maintain stability. I'm just saying that some sort of improvement really needs to be made in this regard. I'm not one to typically say "you must have have max DPS at all times", but when there is a potential for massive DPS loss across the raid group with player skill not factoring in, there is an issue that needs to be addressed......