There was once where my friend came to me asking me to test his guitar telling me his strings were brand new. So I took the guitar, played a bit and looked at him and asked, "Are these .010s and the original gauge was actually .009s or lower?" The answer was obviously yes.
Many a times when many of us who aren't inducted into setting up our own guitars have no knowledge of the existence of different string gauges. What my friend did wasn't wrong but what wrong he did was to miss out another step commonly known to us as setting up. What happened to his guitar was that due to the higher tension of the .010s, the neck warped slightly, causing a pretty high action on the 12th fret. Before you go "Come on, it's only high action", it doesn't end just here. The other side effects of a warped neck are that it causes buzzing, usually from the 13th fret onwards.
So the next time you decide to just change your string gauge, please take note that if you are changing to a higher or lower gauge, make sure that your neck is still straight or a slight bow.
