Accidentally turned on Siri using an iPhone 4 (not 4S)

Solution 1:

Was it Siri or was it Voice Control? Voice Control has been on iPhone since iOS3.

Siri on the left,Voice Control on the right.

You activate Voice Control by holding the Home button for a few seconds, even while the phone is locked.

Solution 2:

Attach your iPhone to a Mac, If you have access, and see if you can get at the crash logs or the device console. Like others, I'm curious to know more about this. If you can see the crash logs via Xcode or something, you might be able to see more about this. Otherwise, it's a bug which you can't reproduce. Oh well.

Being as siri is part of the OS, it's entirely possible that it loaded, then crashed because of missing files or whatever.