My family and I were having dinner at some restaurant when I was a kid, and when the waiter came up to our table to introduce himself, he said, "hey, my name is Mark, and I'll be taking care of you this evening" (or something similar). After he'd left, my sister said, "hay is for horses." My mother just stared at her incredulously for a moment; she'd thought my sister had said, "he's gorgeous."
We all still get a laugh out of that story occasionally.
There are also a couple incorrect expressions I see and hear very frequently that are more annoying than humorous.
[li]"For all intensive purposes" instead of "for all intents and purposes."[/li]
[li]"You've got another thing coming" instead of "you've got another think coming," which is actually understandable considering the original is meant to be a pun and seems incorrect.[/li]
Also, back before Experts Exchange added a hyphen to their domain name, it was very easy misinterpret. There's a list of similarly unfortunate domains somewhere if I can find it. (Edit: Top 10 [http://blog.dreamhosters.com/2006/07/26/top-10-worst-domain-names/] and another 20 [http://blog.dreamhosters.com/2007/01/26/20-more-unfortunate-domain-names/])
