This question concerns a Mac Pro running 10.6 with 2 monitors set side-by-side and configured as one big monitor. Recently I have found that sometimes a "dead spot" appears to my USB Apple mouse in the lower right quadrant of my left monitor. When my cursor was in this area, nothing was clickable (making a window active, scroll bars, buttons, etc.) on that part of the window. Other parts of that window that were out of the dead zone were clickable. I noticed that some of my Safari windows straddled the junction of the two monitors, so when I dragged those windows to be totally in the left monitor, the problem seems to have gone away. Any ideas as to what might be going on? I have had this hardware configuration for several years but this problem is very recent. Thanks.
Check the layout of your displays in the "Displays" preference pane:
Normally it's not possible to position your displays such that there is an actual gap between them (at least on Lion), but perhaps moving them around will fix the problem.