When each character makes his trademark gesture, such as Felix' triumphant laugh, the exact same footage (with voice and sound effects), inserted onto a different background, is often reused from episode to episode to save money.
In the pilot episode The Magic Bag, not listed on IMDB, when Felix is captured by the tractor beam, his yowl is out of sync with the picture. This has been corrected on the DVD.
In episode "Glittering Jewels", not listed on IMDB, a page from a library book says "Ravens steal anything that glitters and leave a bright object in it's place." The apostrophe is inappropriate.