Screen turned off whilst reading email

[ventmore]

I checked my email, and as I put my thumb on the screen to scroll down, the screen turned itself off, just as though I'd pressed the sleep button. Pressing the home button turned it back on, and I (think) I had to enter my code again, just as normal.

The auto-sleep setting is set to never, and it had only been a few seconds since I'd touched the screen anyway.

Has anyone seen this behavior before?

Cheers
Neil