Here is a look at one of headaches.
For example, this. The Shiela and King of the hill animations are full of these, so i have to render a single scene multiple times just to get it right. Its generally on anything longer than 30 seconds. There are multiple reasons why this happens, but I do my best to fix it.
I always do video inspections and quality control, so don't worry about any of these in the animations I release!