It's a learning process!
Haven't really played any games before that had to deal with shoving various commands under the same button depending on the Mode you are in. In Assassin's Creed case, that was Low Profile and High Profile, both of which took some good time (and quite a few 'whoops'es) to learn how to use right. For those of you who have never played it, in Low Profile (holding no button) you can gently push people aside while in High Profile (holding down the Shift buttom in PC versions) you instead grab a person and quite violently slam them to the ground.
Three guesses what I did. No, really, you only need one.
The reason I was prone to accidentally holding down Shift was because you move considerably faster in High Profile.