Especially when replying to a post and including links. This just happened this morning for the first time ("403 Forbidden on this server").
So, you type out a post, click Preview, and you get an error? Does the error take up the whole page?
What type of computer are you using and what browser? What is your input method for clicking the Preview element? (Mouse, touchpad, touchscreen, etc)
A HTTP status code of 4xx is from the server indicating that a request was received that could not be fulfilled. The Preview button is an input element and is not a link. The event handler does make a little server request when you Preview a post, but if this somehow resulted in the display of an error, it is still weird. It should fail more gracefully than that.
A 403 error might indicate that your actions or your browser were doing something unintended for some reason.
The error is coming from the server, not your device, but it is being triggered somehow by your device, and it may be the result of some software issue or unintended action by the user.