This is in response to another post about the Magic Mouse intermittently moving much slower than normal.

I have three Magic Mice and after using any of them for a while, they will sometimes start to react with very little horizontal or vertical motion. It is not related to any one app, and I am very specific about how I set my tracking speed, which has not changed. The only thing that seemed to help was to slap the mouse on the desk and then it would act normal for a while.

After researching possible causes and not seeing anything that related to my symptoms, I used my iPhone to look closely at the IR sender/receiver and found a very small hair, that was crossing in front of the IR. It was almost undetectable, but after removing it with some tint tweezers, this problem that has plagued me for two years is now resolved.