Basically, we’re looking for decisions which had terrible consequences and are generally considered to be awful mistakes, but which, given the information available at the time to the person or people making the decisions, were in fact the most attractive alternatives (or were at least debatably so). There are two ways this could work:
The choice was wrong, but knowing it’s wrong requires more information than the decider should reasonably be expected to have had. (We flip a coin for money, Heads pays $200 and Tails pays $100; it’s actually a trick coin that will always land Tails but, given that I have no way to know that, choosing Heads is entirely reasonably.)
The choice was right, but because of luck or the effect of some separate, unforeseen event it turns out to have worse consequences than an alternate choice would have. (Same as above, except it’s a fair coin that just happens to come up tails – here, no matter how the coin actually lands, calling “Heads” was the right decision.)
Ok, I’ll start: the Munich Agreement. Because we know how things turned out and exactly what kind of leader Hitler was, we see it as foolishness, cowardice, or both. At the time, however, Chamberlain’s acquiescing to Germany’s seizure of the Sudetenland was probably the most responsible course of action. The U.K.'s armed forces were woefully unprepared for war (and they did use the intervening year to modernize and expand). Since it would seem extremely unlikely that Hitler was actively, specifically seeking a general war with England and France (which he was), the assumption that acceding to this demand stood a fair chance of being the difference between war and peace was a reasonable one. And there were, in fact, a high proportion of ethnic Germans in the region who were agitating to be joined with Germany or granted independence.
Most importantly, though, we draw our historical lessons largely from the second world war. Chamberlain et al would have been drawing their lessons primarily from the first, the most obvious one being the importance of not letting relatively small disputes snowball into continent-spanning wars in which millions will be slaughtered. Keep in mind that Chamberlain received a hero’s welcome upon his return to England; had he returned to London with a declaration of war rather than a peace treaty, he would have been lynched on the tarmac (hyperbolically speaking).