There's something a bit screwy going on with the embedding functions. In trying to embed a gif in my post via the url functionality, I found that while the image would appear in my in-process post, if I tried to actually post it or hit the preview button I'd get the vague explanation from the forums that "something went wrong".
I experimented a bit while making this post, and seem to have figured out how to replicate it. For whatever reason, a draft with an embedded gif becomes unpostable when the post also contains an embedded quote and an embedded smiley (not the text, the picture that replaces it). Curiously, this issue can be bypassed by changing the smiley back to text format.