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I get this sometimes too. I just let the editor sort it out and click submit post a 2nd time. It usually works. Copy and paste is limited, because for example you cannot directly paste an image into a msg. You can reference the image to a website that stores your image. Pasting is certainly useful when you are completing the "link" or "inserting image" but only in respect of the URL. You should also be able to cut and paste text into your message but again it needs to be html compliant. If in doubt you can always switch to "edit as html" (button on the top menu bar) this may give you more clues.
Just recently covered 'invalid HTML' in posts in another forum feedback question you may also wish to read that thread (click here).
Essentially what is happening here is, that the other editor is including some HTML tag in the 'markup' that is not allowed in forum posts. Basically, the subset of HTML that is allowed in a post is limited to fonts and formating, image control and link formating tags. Other tags such as those that allow a poster to post malicious HTML are not allowed. As cgoldman points out the forums editor will usually sort this out for you, it just requires an extra check that the post is still formated the way you intended, then posting the message a second time.
[edit: made link more noticable, 3 people let me know they didn't realize it was a link, thanks!.]
Thanks Allen_K! I “read that thread” you suggested. It was very comprehensive. Charles.