what is 'optimistic locking failure' ?

i have been testing my app for core data with icloud in ios 9 and while saving the context after insertion works fine, the execution freezes a couple of times with the message 'optinistic locking failure' followed by a long number. this happens 3-4 times and then the execution follows normally. i dont understand what is happening.


i only have one context in my app and it is on the main queue.

what is 'optimistic locking failure' ?
 
 
Q