Sometimes, it's the "o" key, but lately it's been the "m" key.
When I hit the "m" key, either by typing a message letter by letter or using Swype, my editing stops. I cannot recover without closing my editor and getting back in. Sometimes, I have to completely reboot my phone, and that seems to help. When I get back into the message to try again, I can edit until I hit the "m" key again.