Spare a thought for Google Wallet users who have learned this the hard way, but at least you won't have to fall into the same little-known trap. Essentially, if you have a Galaxy Nexus, HTC EVO 4G LTE or any other compatible handset and you perform a factory reset, there's a high chance you'll spoil the secure element that is designed to protect your NFC chip from tampering. This disables Google Wallet and it seems there's no fix once that happens, other than calling up a friendly (and hopefully interactive) customer service agent. However, there is a way to prevent it: before factory resetting, you have to go into the Google Wallet app, choose 'Menu' then 'Settings' and 'Reset Google Wallet'. There it is -- simple enough, but evidently not advertised enough by Google or the phone manufacturers themselves.
Samsung Galaxy Nexus