(Classic) Tanks randomly drop down to 0% health
From what I can tell, this seems to happen when the tank is being hit by 5+ mobs. VuhDo will show them as having 0% health even though they're actually 80% or higher. Happens multiple times every single raid night, and it's almost always on large 5+ mob pulls. The tank will drop to 0% for 1-2 seconds before going back to the proper health value.