I spent too much time fishing around for the source of this error. It appears to be a bug on Apple's side. If you're using Storyboards and have a view controller with a UISearchDisplayController on it that you've dragged in using Interface Builder you might see this message under low-memory conditions.
[UISearchDisplayController retain]: message sent to deallocated instance
It'll happen when you navigate from the view controller with the search display controller, receive a memory warning, and then return to that view controller. I found this while testing under simulated low-memory conditions.
Finding this now is good; with every release I want to believe users will rarely if ever experience a crash.