Hmmmm....this is something I just discussed with a group extensively yesterday...
While I believe that gifts should be given from the heart and "in the moment," I understand that feelings and relationships can change and resentment or regret may develop at a later date for giving a gift. Even a "non-sentimental" gift may become sentimental after the passing of time, as Ansley stated.
One gift that I do regret was giving my friendship and trust to somebody who completely broke my heart, twisted my words, held my realness and vulnerability against me and completely dropped me out of the blue.
This woman was someone who was very open with me as our friendship was developing, or so I thought...yet once I shared almost everything about myself with her, our friendship was suddenly over and she began gossiping to other people about me and my "many issues."
At the same time, however, she taught me to be a bit more guarded about what I share and with whom, and I am much more particular about the people who I let into my life. So perhaps I received a much more meaningful gift in return?