Early in the cartoon, Bugs is tunneling his way to San Francisco when he hits his head on a rock. As he lifts and rolls it out of his way, the background is visible through it, and it becomes apparent that only the rock's outline has been painted. The error is only noticeable for a few frames, when the rock overlaps a part of the background that isn't the same green as the surrounding grass. It's likely it was meant to be filled in with the same green as the grass; because it looks filled in when it's over that area, this is probably the reason for the error.
When Nasty Canasta is marking his cards with 'Invisible Ink', he never looks at any of the card faces to see what card it is before he marks it.
The 'bank' facade that Nasty Canasta makes reads as follows: Top: 'Bank and Safty Depossits' Bottom: 'Nasty Canasta Prez. 70% intrust Pade Quarterly'. Note that, in actuality, all the 'N's are flipped. Yet, in spite of all of the incorrect spellings, he manages to spell one of the most difficult words - 'Quarterly' - correctly. In reality, it should have been spelled 'Korterly'.