My website sells widgets.
My site also has a few pages on bouncy balls. We don't sell bouncy balls though. It's just a hobby.
I recently setup a Yahoo! marketing campaign targeting widgets and many variations.
At no point does "bouncy balls" or anything similar show up as a keyword in my campaigns.
Guess what - my ad is showing up for "bouncy balls" searches - complete with keyword substitution.
Our site sells all kinds of widgets.
Come check us out.
My assumption is that this behavior is part of the "Advanced" match type.
I've disabled it for now - and am keeping a very close eye on future traffic from Yahoo! marketing.
In general traffic quality has been OK - but this kind of thing is a little alarming. I could just imagine it "finding" some other high traffic keyword on my site and then getting oodles of unprofitable clicks...