If you hit this, then you've leaked one or more objects of the type specified by the 'OwnerClass' template parameter - the name should have been printed by the line above.
If you're leaking, it's probably because you're using old-fashioned, non-RAII techniques for your object management. Tut, tut. Always, always use std::unique_ptrs, OwnedArrays, ReferenceCountedObjects, etc, and avoid the 'delete' operator at all costs!
References DBG, juce::LeakedObjectDetector< OwnerClass >::getLeakedObjectClassName(), jassertfalse, juce::LeakedObjectDetector< OwnerClass >::LeakCounter::numObjects, and juce::Atomic< Type >::value.