It's happened to both me and my girlfriend. It's not very common but seems to happen sometimes when software is updating when it has a good signal/wifi and you move into an area of no signal. Having been caught out a couple of times being unable to access important apps because of it (one time being in an unfamiliar area with a half downloaded Google Maps and insufficient signal to download the app again), I just don't risk it any more... Which is a shame as otherwise it makes sense to stay up to date obviously.
I did report this as a bug to Apple, who requested a device log of some kind when it next happens, but it's hard to reproduce manually.
Probably a practice best to run iOS updates fully charged, plugged into power and over stable Wi-Fi. Also an iCloud backup beforehand and an iTunes full-backup where possible seem like good ideas.
Hmm, while I do update outside the house/office, e.g. when I'm bored at some place, I never update apps on the move (e.g. in a car, walking etc), so that could be a factor...
I did report this as a bug to Apple, who requested a device log of some kind when it next happens, but it's hard to reproduce manually.