Trying to track down cause of "Warning: observer object was not disposed manually with Dispose()"

I've been trying to track down for a while the cause of "Warning: observer object was not disposed manually with Dispose()". Every time it appears the app promptly crashes. It always happening following the one view controller being replaced by another, in that the root view controller of the window is swapped. Has any one else come across this message, followed by a crash? If so any hints on tracking down the cause?