Was the magic mouse losing connection previously? Do you have a 2.5/5Ghz router nearby? Are you using the recent MBP model?
We have received feedback about this issue from our user group also. The last few generations of MBP Bluetooth has had issues with 5Ghz interference. Some routers were too close and people had to disable the 5Ghz and the trouble went away entirely. Apple isn't doing anything about it and just published an advisory. Click the link below for more information from Apple.