If you've ever had your app rejected, why? And also, how surprised were you by the rejection? Did you know you were doing something questionable?
2.12: Apps that are not very useful, are simply web sites bundled as apps, or do not provide any lasting entertainment value may be rejected.
The first, I sort of expected - it wasn't terribly useful, but it was free and reasonably pretty, so I thought it might get through. It was my first app. Oddly enough, my second app was basically a theme on my first app, with the added advantage of an extra button that started and stopped the action. It sailed through.
The second is a great little baby rattle that every single baby/toddler I've given it to loves and plays with for ages, but Apple just didn't dig it. I was a bit surprised, especially given the number of other apps out there that are similar in functionality that have clearly been accepted.
I revised it once and added in some extra features and options, but it got rejected again for the same reason. My daughter still asks me if she can play with it about once a week, so I can't complain :)
Good luck on the submission - my best advice is submit and get straight on to either developing the next app or enhancing this one for a version 2 - it can take a while to hear back from Apple and you can waste a couple of weeks hitting the refresh button in gmail instead of working on your next app.
Another rejection was not testing for network availability (connection), when my application requires network connectivity to work.
Also rejection early on for using undocumented API (that allowed app interface to be viewed on external display). Even though the code was commented out in the submitted app, it caused a rejection.
In all cases after fix/resubmission, app was approved (2 weeks each time).
There's a spot when you submit where you can add notes for the reviewer, that's where information like that goes.
It was for allowing multiple popovers to be visible at once in an iPad app.