One of the advantages of using the Facebook Chat client comes from how easily and intuitive it is to use: to send a chat message, you just type a few words and hit the Enter key (on Windows or Linux - "Return" key on Mac OS X). The process takes a couple of keystrokes, and before you know it, you have a full chat conversation going on with another or multiple Facebook users. The downside of this ease of use is of course that you can accidentally send a chat message to the wrong person, or inadvertently send something you did not mean to say. This tutorial discusses the options you have in those cases, and how to recall and unsend Facebook Chat messages to the extent that you can, and try to cancel chats after the fact.
Here are a few scenarios where you can prevent a chat message from reaching other Facebook users: but first, look at the mini Facebook Chat popup on your own profile. If the comment you want to cancel is still displayed inside the input text field, as shown on the screenshot below, just erase it and forget about the whole thing - you were lucky enough to realize your mistakes before actually sending a wrong message, or a chat to the wrong person. Just like it is a good idea to close your car doors only while holding your keys in your hand, it is a good idea never to hit Enter / Return before re-reading the chat you're about to send!
...And these are your options to cancel and try to undo a Facebook Chat message sent by mistake.