At a guess, I would say that your IP address may have changed while you were writing the post, so that when you actually attempted to post it, it would have rejected it.
Perhaps your connection dropped, which would cause a change of IP, that is more likely to happen on a mobile connection.
Normally a draft copy is saved by the system, so when to attempt to post again, you are given the option to reload the draft.
I am not sure whether that is the case for the mobile version of the forum as well.
Its best to keep posts short anyway, otherwise people tend to ignore them.
I often see the same error message, but my external IP address definitely hasn't changed immediately before, during or after posting.
Also, the last time it happened, I'd actually written an essay, and the saved copy only contained 2 words:(
(it was a post where I'd edited it to state "complete crash" or words similar).
I do know that Lithium store the IP address used at the time the post was made, plus the user ID for 2 weeks.
(this could by how multiple acc's are discovered;))