Avraham's "sister" Sarah
That Avraham would so easily hand his wife over in either case is a problem. But the second incident is especially problematic for a few reasons. First, he's been down this path before, so the first incident didn't cause him to rethink the approach. Second, we are to believe that a man who rescued Lot from his captors, who was victorious in war, and who has found favor with God (not that he should rely on miracles) cannot protect his own wife? Third, he could have avoided the problem entirely; unlike in the first case where he had a specific reason to go to Egypt, in this case nothing in particular directed him to Avimelech's land; he could have gone anywhere in Cana'an. And, fourth and most important to me, this was clearly pre-meditated; the torah tells us that he told Sarah to agree to the ruse before they entered Avimelech's territory.
Someone at torah study argued that you do what you must to survive if it's not a violation of the "big three" (you can't commit murder, idolatry, or sexual immorality to save your life). But we are not talking about the case of bandits setting upon you from out of the blue and demanding your wife or they'll kill you, and you have to react in the moment without thinking things through. Against the possibility (not certainty) of this happening, Avraham plans a deception to give up his wife up front. I would have expected some of the commentaries to call him out on this.
Of course the torah shows us the patriarchs and other leaders as real people who make mistakes and don't always behave well. It's one of the things I like about the torah; we don't claim that everybody is a saint. So maybe the act is so obviously wrong that the commentaries don't feel the need to mention it -- we can learn the negative lesson without help? On the other hand, commentaries sometimes latch onto some pretty minor points. And it's not obvious to at least one person who was at torah study yesterday... hmm.
[1] Yes yes, I'm aware of his claim that she's actually his half-sister so that's not really a lie. Clearly his intent is to mislead and he is omitting a pretty important fact.

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http://fourmothers.files.wordpress.com/2010/10/take-my-wife-please.pdf
I know a lot of people don't like Reis' readings of scripture, but some do seem plausible to me. E.g. the theory that Yitzchak was a diabetic (impotence leading to taking advantage of a rare arousal [sporting in the courtyard]; blindness and needing to be served; preferring meat; other things which I forget).
Here, it is suggestive that the last pasuk of the first aliyah justifies the deception with first: that it go well for me because of you [i.e., I get money], and only second: that I survive because of you. Enriching himself as a result of the deception is premeditated.
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Um, isn't asking your wife to claim to be unmarried and therefore available for another man the same as asking her to encourage adultery (and presumably to cooperate if/when it happens)? How is that not one of the big 3?
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