iPhone app crashing in 4.0 when switching between apps in background

I have an iphone application which has to support multitasking. But when i open the app and do some work then close, it is going to background and when open different app and then again switch to my own app it is crashing..

Don't have idea about when i did mistake..

it is giving error like this:

[__NSCFArra开发者_如何学运维y removeObjectAtIndex:]: mutating method sent to immutable object

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You try to remove an object in an immutable array, it doesn't seem to be a multitasking problem..