"In heads-up hold'em (either limit or NL), one thing that really puts a smile on my face is when the preflop button raiser checks behind on a ragged ace-high flop.
This makes as much sense to me as the 3rd-street raiser in razz checking on 4th after catching a second baby. If you actually saw someone do this in razz, you'd immediately think, "fish". Yet people do this at the highest limits in hold'em, and it pleases me.
Now, one day when we compute the optimal strategy for hold'em, it may actually turn out that it is correct do this with some weird mix of hands (like AA, KK-JJ, and 3-to-a-double-gap-straight-flush or something -- feel free to speculate on what you think this range might look like), but I'd guess no one is particularly close to figuring this out yet."