Lupin's history with Dementors

[captain_suburbia]

No: HPFGUIDX 100638

I hope this isn't a repeat (I have looked!)..... 

In PoA when Harry and Lupin are discussing Dementors, Lupin says
something to the effect of "I don't pretend to be an expert at
fighting Dementors, Harry - quite the contrary...."  (page 141,
Australian edition)  So, what, has he had an incident before?  Kind of
implies that he's failed to do so in some way, doesn't it?  Any
thoughts, theories?