I can confirm that the camera shutter button on the outside is sometimes a pain on the Z520. It's active even when the phone is closed in order to enable a self-portrait mode utilizing the external display. Cute, but all but worthless to me. There is a delay of a few seconds before holding that button puts the phone in camera mode, but it is easy to grab it by that spot. Once in camera mode, pressing that button again takes a picture without delay. I've not had a problem to such a degree that it takes pictures while in my pocket, though I have fished it out of my pocket to find that I've grabbed it by that button, then shifted it in my hand and snapped a picture. Stuff like that.

It does seem like a notable oversight. I can't be the first one to notice in about 10 minutes that it would be nice to reprogram that button to something more useful. That said, the other positives are still outweighing the negatives for me.