You may sometimes receive reports from your customers of problems that are hard to troubleshoot over email. It can be a pain to really get to the bottom of the problem without access to the app data stored on the customer's device.
There is, however, a way your customer can send you that data: by syncing their iPhone with their computer and sending you the app's data file in the iTunes backup.
To do this, you'll need to know the name of the app's file in the backup. If you have a look in your own iPhone backup folder, you should be able to find it with a string search tool such as grep by searching for a string you know will be in the file.
Once you know the name of the file, simply ask your customers to send you their copy of that file when they have a hard-to-diagnose problem. An easy way for them to get the file is for them to extract it using a tool such as iPhone Backup Extractor.
If you have any other good tips for debugging tricky iPhone app issues, then do share them in the comments. Happy troubleshooting!