Ah, the old mysterious error code! You would think it'd be easy for OS, web and software developers to put more helpful information in their error messages, but the truth is it's not always clear what's specifically causing an issue. Google Play is no exception, with its fair share of cryptic missive for unsuccessful operations. Fortunately, you can usually fix these problems yourself after being pointed in the right direction.
LostInTechnology's Furqan Shahid has compiled a list of the more common errors you'll hit when trying to download apps from Google Play, make in-app purchases or grab updates. A majority of solutions are similar, involving clearing a cache or deleting files and trying the operation again.
Errors such as "491" require a bit of extra work, in its case, removing and re-entering your account details.
It's tempting to nuke your device from orbit (read: do a complete wipe and firmware reinstall) when issues like this crop up, but it's worth troubleshooting for 5-10 minutes before taking such drastic measures. At the very least you'll learn a little more about the inner workings of your gadgets (or just become immensely frustrated... depending on your patience and curiosity levels).
How to Fix Infamous Google Play Errors [LostInTechnology]