My teacher mentioned a volunteer project in the rec-letter that is not on my common app activities list.
There should be no problem with that, but is there a reason it's not on your list? If it was meaningful enough for your teacher to write about it, maybe it should be there, if you have space. If you feel that your teacher's explanation of the project was enough, then you can certainly still leave it out, but if you there's more of a story to tell, feel free to add it.
Hope this helps, and let me know if you have more questions!
I don't think so. If anything, it shows that you do more than what you can list on a piece of paper. I would see this as a positive. More than likely the admissions team reading the letter won't question it too much.
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