You can't outbid yourself on YJ or eBay.. the 
only time you raise your own bid is a) when you 
hit a reserve, the high bid will then jump to 
the reserve.. or b) possibly when your max is 
the current highest bid, but below the next 
legal increment.  So if you raise your proxy, it 
may jump to the next increment.

Otherwise, you shouldn't be able to raise the 
current high bid by changing your max.  Maybe 
the other bidder placed a bid right when you 
raised yours, making it look like you're the one 
who did it..?