Several Rock Band players are dealing with an issue with the Fender’s strummer bar. If your Strat starts going wonky and doesn’t register strums or occasionally double-strums, chances are your problem can be easily solved with a couple of screw drivers.
The problem is that the screw which holds the switch in place can come loose (or was assembled loose at the factory). Over time the entire switch mechanism can rotate out of place. This can cause the switch to stick in a closed position, or close multiple times when the strummer bounces against the untensioned switch.
This is actually a pretty common problem with a lot of different controllers. I’ve had the same issue with old joysticks and even a few RC car controllers. Whether it’s a simple switch, or a control that rotates a potentiometer, if the controller is used frequently and forcefully, the adjustable mount for the sensor can wiggle out of place. It’s just a matter of cracking the case and poking around to see what went wrong and correct the problem.