I never give a sub a task I know she is going to fail. What's the point? Some people might be into that but if I give someone a task I want them to succeed. If not a punishment might be in order but to punish someone for not doing the impossible is not constructive at all.
Now, no Dom is 100% perfect. We do make mistake and overlook things, of course we try not to but it can happen. If I gave someone a task that was in fact impossible to perform I want to be informed of why so that I can change it to make it possible. I would in fact be disappointed if a sub did not inform me right away.
Of course, a task might be designed so the sub would think she would fail. Then she is probably in for a surprise...