I often land on answers and realize that, at some time in the distant past, I already found this answer and read it, and voted on the question and/or my favorite answer. I then re-read the answer, and am suprised because I have already completely forgotten it. I wonder how long ago I found this the first time?
I know that I could go to my user page and sift through EVERYTHING I have ever voted on and find the date, but that is dozens of pages, with no search. Currently, hovering over the vote button after having already voted gives the hover text "This answer is useful (click again to undo)". What if it said "You marked this as useful in Jan '14 (click again to undo)"?