It's probably a better question for http://communities.vmware.com/community/vmtn/suggest/newsite but I suspect that well, an error ocurred while talking to the server Why exactly the forum needs to do that is a good question, but it's probably for autosaving. Occasionally after composing a post, I decide not to finish and cancel. If I come back to it later, I've been offered a recovered version a few times.
I still don't trust VMware's forum composer. I always select all - copy the text before clicking Post Message. This is the only way I'm guaranteed not to lose work.
I've lost too many paragraphs of text "trusting" that the forums will save whatever I'm composing as I go along. IMO, if the auto-save "feature" can be turned off maybe the the load on the entire system could sustain the traffic the community site gets. For me it loses more data than it saves.