Mention that you have a flaw, but then also add a suggestion of improving that flaw or demonstrate any actions you are taking to address any weakness. The key to answering questions like this is to show you have learnt from your mistakes and are taking concrete steps to address it.

Occasionally I have been told that I take longer than other colleagues to complete complicated projects or tasks. But this is only because I want to make sure the work I do is to the highest standards.