There was a mistake at the rose ceremony once...The Bachelor, Season 5, with Jesse Palmer as the bachelor. Episode 1 and so many names to remember. Or forget, as the case may be.
Here is a link to the RTVW summary for that episode that was hilariously written by Greeneyes:
http://www.realitytvworld.com/index/articles/summary.php?i=247
Well deserving of being dragged up from the archives!!
My guess is that since then they do something to help prevent this sort of thing happening again.