The forum does reject some posts with what it considers to be funky characters, and doesn't tell you that's the reason. Oddly enough, exactly what characters are allowed varies depending on exactly how you access it. If you come in via rv.net on an unsecured connection (http), for example, it will reject the percent sign, but it's okay if you use a secure connection (https) to rv.net. Some other more specialized characters are always rejected, like em and en dashes.
At any rate, if your post is rejected, it can often help to hunt down any characters that are the least bit odd or unusual.