Often, I'll see an opportunity to introduce a little ambiguity into one of my figure drawings. This usually happens during the sketching stage, and when I do see it, I twist it as hard as I reasonably can.
Ye olde classic painters would imply ambiguity all the time, although usually it flies right over the heads of viewers who don't know things like 'that tiny dog at the foot of the bed symbolizes fidelity'. (Yay art history training. *waves a tiny flag*)
In my case, I tend to do gender and relationship ambiguity, because I don't like when people jump to conclusions about these things. I just want to give these assumptions a good-natured poke with my pointy drawing stick, and hopefully get people to re-examine their frame of reference. (Ha! It's funny because my drawings are like a pointed verbal comment, but the pencils and pens I used to make them are ALSO pointy! That's pretty clever for being on a double-dose of Claritin!)
Sometimes it doesn't quite work though: I just about snarfed my coke after I posted the main art from this post on a art sharing website, and the first community comment was 'He needs more scars.' Leaving aside the overdone trope of the aesthetically pleasing asymmetrical facial scar on the mysterious loner, this is a 'he' ?! ... My, what wide, childbearing hips that fellow has.