Answer by Graham Dunnett

http://answers.unity3d.com/questions/359835/internal-error-too-many-pairs-created-new-pairs-wi.html ? http://answers.unity3d.com/questions/43599/what-are-pairs.html ? http://answers.unity3d.com/questions/662062/ontriggerenter-with-multiple-instances-at-same-loc.html ? http://forum.unity3d.com/threads/ontriggerexit-bugs-internal-error-too-many-pairs-created-new-pairs-will-be-ignore.125451/ ?