It's not so much a matter of being careful...it's just something that happens. No-one can ever erase ALL the mistakes in any movie, story, etc. All you can do is hope that they are not major mistakes.
As I said, I personally think this sort of goof by Disney is one of the more fun ones, because it's so minor it's nearly pointless arguing about it, but it creates an interesting issue of how Simba looks with or without the whiskers.
I mean, if Disney had fixed it up, this thread would never have gotten started, and we would not be having so much fun debating which way works better
