was mutated while being enumerated

抛出异常：

2013-11-19 09:06:32.784 LotterySteward[1641:60b] *** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSArrayM: 0x146deee0> was mutated while being enumerated.'

*** First throw call stack:

(0x2f2dce8b 0x395d76c7 0x2f2dc979 0x2dc3ad 0x2dbce9 0x2dbdd7 0x2db4dd 0x1215b5 0x13a2b9 0x2fcc0fdb 0x2f2a7f27 0x2f2a73ef 0x2f2a5bdf 0x2f210541 0x2f210323 0x33f472eb 0x31ac71e5 0xccb1b 0xccaa8)

libc++abi.dylib: terminating with uncaught exception of type NSException

截图如下：

网上查了下，大概的意思是使用了多线程，对一个NSMutableArray对象进行修改操作时报错了。