Shulchan Aruch, OC 692, says that if someone reads m'gilas Ester without reciting the benedictions before and after it then he fulfills his obligation of reading Ester. It doesn't say what to do about the benedictions, however. I wonder about "sheasa nisim". Is there some way to say it after the fact?
Someone who cannot light Chanuka lights says "sheasa nisim" if he sees someone else's lights. What if he already satisfied his lighting requirement, but skipped saying that benediction? Is there some way to say it after the fact?