The little girl gets kidnapped - twice
I love Sherlock Holmes and I think that this cartoon did a great job of showing him to children. For me as an adult it was entertaining but not a good enough Holmes movie. And furthermore Basil did make huge mistakes in this film.
First they take the little girl with them on their search for the kidnapping bat. Off course it goes wrong. They knew that the rat :-) had her father because he wanted him to build a toy. So they knew that the daughter could have been used as extortion - which she was!
Then they take her to the queen's palace even though they know that it's filled with bad guys. And again they fail to watch over her. Even her father was doing other stuff while she was being kidnapped. So 2 times in one day they took a little girl to a place where they knew that there would be bad guys. She was the rat's biggest asset throughout this movie. If she would just have stayed at home the rat probably couldn't have forced the father to build a toy queen, or caught Basil while he was looking for her, and he couldn't have kidnapped her again and put Basil in danger, again.