I might just be overthinking things...but today while I was at research, my PI and I were just chatting about various things (towards the end, when I was about to leave). She mentioned earlier in the day that she believed she could write a LOR that could definitely help my application. But later on in the day as I was wrapping up, she off-handedly mentioned that she thought while I wasn't bad, I was rushing some of my work and advised me to slow down, since I've made a few mistakes before. I'm afraid that she now thinks that I'm careless--or that she doesn't think I was as good as the last student there. I've always planned on asking her for a LOR (I've been with her for a year now, and we seemed to get along well), but the comment today scared me a bit. I'm definitely going to "work slower" and be more careful, but I'm scared I may have messed up her image of me already.
I'm assuming that you worked with your PI for months or even years? First take a few deep breaths. Now ask yourself if you really think you screwed up so badly that in just a couple of hours you managed to destroy the reputation that you built up over the course of hundreds of days. (Hint: The only way you could have done this is if you literally slapped your PI across the face.)
If she mentioned she could write you a LOR, then yes. Definitely ask. They want what is best for you, hence the constructive criticism. If your PI did not care, she would have had a more stern "talk." You're golden, ponyboy!
