Well, to answer my own question I do seem to have an unresolved
journey but it isn't yet recorded as that on my Oyster - I took my
Oyster to the Tube ticket office and obtained a printout. For Tuesday
it records "Pre Pay Entry Green Park" and has charged me £1.50.
However, the bloke didn't seem to be able to do anything about it - he
suggested I try in a few days and see if the journey description
and/or cost had changed.

I did have an unresolved journey on my ticket for 15 October last year
- it recorded "Unstarted - Oxford Circus" but again only charged me a
single Tube fare, £1.70. I got this resolved, I have a sneaking
suspicion that this should be a day when all my tube journeys were
capped at an ODTC but of course this long ago, nothing other than my
£20 top-ups and the unresolved journey are shown on the Oyster Usage
Statement. It's only £1.70 so what the hell - but if you could read
your Oyster statement online, it would help keep on top of thee
things.

So presumably, at some time the description for Tuesday's journey will
change to "Green Park - Unfinished" and I can ask to have it resolved
- but I'm slightly surprised that it doesn't seem to charge you a
default ODTC price on an unresolved journey.