Ignore last comment -- I was being stupid.  I can reproduce it and now I have
added logging it turns out that this is a much more frequent problem than I had
realised (at least 10% of my events hit it).  The result is that for events
which have this problem, the event local time (e.g. 16:00 India time) is
re-interpreted as though it were 16:00 UTC (because the VTIMEZONE is invalid
and is ignored).

I have a fix and am testing it.