It's sometimes funny, but mostly in a cruel way, to see someone fail to get a joke:

Ha ha.

Oh. Uh ... sorry.

Maybe the best thing is to pretend no joke was being made.

But the other day I was trying to find out if Moses Hadas indeed wrote the gag often attributed to him, "this book fills a much-needed gap", and if so, what book was he talking about? If you search the phrase on Google you get "about 19,000" results. Sadly enough, "about 18,995" of these results are from reviews that use the phrase straight, in its apparently positive meaning. That means most people are heart-breakingly naive. Notice that they didn't just hear the gag and miss it, but turned around and repeated it in a public forum, like somebody in a sitcom. People are duped and it's very sad. Although it explains a lot.