Although I still have not reproduced this problem, I do not have any reports of it in version 1.2.1 or higher so it appears to be fixed. I recommend you continue to backup your data regularly.
I have heard from a few people (about 1% of the people who purchased Attendance2) that after using Attendance2 for a few days, they can no longer open their data file (it hangs with the semi-transparent message that it is opening the file). When they back up the file to Dropbox, the data file is essentially empty (it seems to be 22 bytes which is just enough to indicate the directory structure of the data, but no actual data). So it appears the app is not correctly saving their data.
I have been using the app since January in my classes and have never had this happen so I'm having trouble tracking down the problem. I certainly wouldn't have released the app if I was aware of this since it's not worth the stress for the small amount of money I make from the app. So until I get this figured out, I have temporarily pulled the app from the store. I will be using one of my two "support tickets" with Apple to try to get a number of questions answered to see if those answers can help me track it down.
I am using a new feature in iOS 5 to store/save the data so it's possible the bug is in Apple's code (which would mean there's nothing I can do to fix it until they do) or perhaps I'm not using the code correctly (which is the more likely case) or perhaps the people that are having the problem are doing some combination of steps I had never tried and is leading to the error.
In the meantime, if you are using Attendance2, please backup your data every day. When you are done with your class, use the option to Backup to Dropbox or Backup via email to backup your course. When you are back at your computer, check to see the size of the file. If it's 22 bytes or less than 1KB, something went wrong and you will want to access the backup from the previous day. If you're backing up to Dropbox, you can login to their website (dropbox.com) and restore an earlier version of the file to Dropbox and then import that back into Attendance2.
Unfortunately I have no way to contacting people who purchased the app unless they have contacted me so if your colleagues are using the app, please let them know and tell them to backup the data every day.
If anyone knows of steps that make his happen consistently (i.e., you can take a file, do certain tasks with the app, and it happens - and then if you restore the previous data file and do the same steps, and it immediately happens again, please let me know).
I apologize for this, but since it never happened in seven months of testing, I had no idea there was a problem.
I will post an update when I figure this out and when the update is approved by Apple.
I still have not reproduced the problem myself, but I have confirmed that Apple's auto-save will always save the data (even when you close the app or it enters the background) so I have removed the manual save I was doing when an app entered the background. My theory is that both of these savings being done when the app entered the background was leading to a race condition that resulted in an empty file being created (the zip structure itself is the 22 bytes). I have a few people that had the problem beta testing the update and if it seems to work ok for them after a few more days, I will submit the update to Apple and make the app available again after Apple approves the update.
Once the empty file is created, no version of the app will be able to open it so please send yourself reports and backup the data each time you are done taking attendance for a class until we are fairly certain the bug is fixed. I recommend you send yourself a report each day at the end of class. See the Report Settings and turn on the various options that you are using in the "View in Reports" section. Make certain the Start Date and End Date at the top of the Report Settings screen include all dates you have entered and will enter. You can send yourself the Spreadsheet Report or Detail report to get the data onto your computer. At a bare minimum, on the screen where you can take attendance, press the Communicate button (if you are on an iPhone or iPod Touch, the row of buttons at the top scroll left/right) and select Email report so you have your basic status records for that day.
After reading through more of Apple's documentation, I think I might have found the cause of the problem. I have submitted an update to Apple tonight and requested an expedited review. Once Apple approves it, I'll put it back in the store (assuming neither myself or the people beta testing the changes see the problem again). Note the updated app cannot recover a bad file (since the file is empty) so please make certain to backup each day and use the reports so you at most lose one day's data and with a report you can re-enter it. I will post here and to Twitter (@davereed) when the update can be downloaded.
Version 1.2.1 is available in the App Store. Please update to this version now if you are using an earlier version. I cannot be certain this fixes the problem since I still have never had it happen to me, but I did make some changes in when the call to Apple's code to save the file occurs. Please continue to backup your data each day after you use it and let me know if you have this problem after updating to version 1.2.1. I cannot fix previous data files, but if you open a course with version 1.2.1 or higher and then later cannot open it, please contact me at firstname.lastname@example.org.