And I've noticed it can happen if you're sending a mesg & someone else has sent you a message simultaneously can error it out or if using a different messenger other than the stock app such as text secure, which I haven't been able to get MMS working on that it's strange there
I've had this problem on occasion and I'm currently on stock. For some reason, I've solved this by uploading the MMS to Dropbox and sending by DB. Weird. Actually, since KK update, no problems (knock on wood) so far.
Since TextSecure has data channel Push Services it helps With using Mms vs entering the Mms URL,
Proxy & Port Manually But you Putting a lot of Trust in Moxie Which at least To me Feels like I have a Choice in Privacy Vs knowing every SMS is read & since will be the default in cyanogenmod a whole community does