I'm surprised to find that many mistakes, because I didn't expect that. Usually, I don't notice those mistakes in TV series or movies.
On the other hand, I don't know how many cels there are in an episode or in the entire series, but I'm sure there are thousands of them. 2 or 3 mistakes per episode is not a lot, and they aren't huge mistakes if I remember well, except for the background disapearing in Uran Falls in Love!

I had seen maybe 2 of them in the entire series the first time I watched it a few months ago.
I'm thinking that we could make an interesting comparison with computer programs, like how many mistakes one can find per X number of lines of code!

Maybe they decided they could live with a few mistakes here and there because it wasn't worthwile to spend too much time checking everything? Or maybe they simply couldn't afford it. I wonder if they could easily spot them when they watched the series!
There are episodes in which I didn't find any mistake so far even if I paid attention: Atlas Lives Again, Save the Carolina 3, The Secret of Bee City, The Genie From Outer Space, The Monster of Clarken, I think The Light Ray Robot and probably a few others. That doesn't mean there aren't, of course!
