Beware UserDefaults: a tale of hard to find bugs, and lost data

Excuse the alarmist title, but I think it’s justified, as it’s an issue that’s caused me a ton of pain in both support emails and actually tracking it down, so I want to make others aware of it so they don’t similarly burned. Brief intro For the uninitiated, UserDefaults (née NSUserDefaults) is the de facto iOS standard for persisting non-sensitive, non-massive data to “disk” (AKA offline). In other words, are you storing some user preferences, maybe your user’s favorite ice cream flavors? UserDefaults is great, and used extensively from virtually every iOS app to Apple sample code. Large amount of data, or sensitive data? Look elsewhere! This is as opposed to just storing it in memory where if the user restarts the app all the data is wiped out.