I had the exact same problem where my HTC Sensation would occasionally take screen shots when I pressed the home button.
At first I thought it was a hardware issue with the power button, but I soon realized it wasn't because if it was, the phone would constantly prompt me to turn off/restart my phone, which it didn't.
What I did notice is that the screenshot error only occurred when my RAM was running low at around 80mb. I fixed this by rebooting my phone, but that only lasted until the RAM ran low once again. The only way to free up ram was to manually kill background apps via the Task Manager.
I was able to find an app killer program that automatically optimized RAM by killing unused apps.
While this hasn't completely eliminated the problem, it's GREATLY reduced the number of screenshot errors.
Android version 2.3.4
HTC Sense version 3.0
Software number 1.45.666.5
Hope that helps.