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I am getting a very strange and very occasional, random crash on a secondary thread. I don't know what to make of the stack trace. Any ideas as to what could be happening here would be appreciated please?

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This occurs when drawing strings and images to a graphic context on a background thread.

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Can you show the stack trace? –  user1118321 Aug 21 '13 at 4:56
Lacking more evidence -- like an actual crash log or real backtrace from a debugger -- this is only conjecture: the symptoms described would generally indicate updating the UI from other than the main thread. –  bbum Aug 21 '13 at 5:08

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Finally got it - this was caused by calling some of NSString's drawing methods to a graphic context on a secondary thread. They are supposed to be thread safe but it is an iOS6 bug. Can be solved by using Core Text or NSAttributedString drawing methods.

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