I took a preliminary look at the November 2008 San Francisco RCV elections.
There were 7 Supervisors elected. 3 elections found a majority in the
first round.
In the remaining four, no majority was found after transfers, I
haven't looked at all of them in detail, but one was on the order of 40%.
In no case did the first preference leader fail to win.
This is seems to be typical of nonpartisan IRV. So far, no comeback
elections, excepting a partisan election in Pierce County,
Washington, out of what must be approaching forty elections.
This appears to be normal: most of the time, when a majority isn't
found in the first round, no majority is found after vote transfers
(except for the "last round majority," which isn't a majority of the
votes and is a mathematical certainty from the method, which
essentially discards all votes which aren't for the top two).
Further, in nonpartisan elections, the preference order in the first
round tends to be maintained after transfers. No reversals have been
seen. In Top Two Runoff elections, reversal takes place roughly
one-third of the time.
I have the ballot image files for the "instant runoffs" from San
Francisco. Rather irritatingly, they don't have the files available
from the districts where there was a first round majority; but those
results might be in the overall result file that I also have, I
haven't looked at it yet.
San Francisco could have replaced their prior TTR system with
Plurality; the results would have remained the same. However,
previously, there were comeback elections where the runner-up did
win. IRV eliminates that, in nonpartisan elections, almost entirely.
I say "almost" not because I've seen an exception, but because the
vote gaps do reduce, sometimes, and in a very close election, a
"comeback" could occur. It's clearly rare with IRV.
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