Theoretically, someone who gets 49% in the first round, could get overtaken by someone who got far less, due to second, or third votes being counted.
But what it would mean is that the 51% of the electorate remaining have expressed preferences which mean that they'd want ANYONE but the 49% candidate, and in my mind it's much fairer that that preference gets heard.
For the record, I don't like AV. I'd much prefer the STV system they use in the Scottish local elections, which among other things lessens the chance of a vote being "wasted". However, it's the only choice being offered to us and I really want to get rid of the first past the post system.