There’s a very annoying bug in the mobile Gmail. You can find it in the mobile Gmail site for smartphones, the Gmail app for iOS and the iGoogle gadget for Gmail (all of them are actually the same app).
If you’ve added multiple sending email addresses to your account and you receive messages forwarded from other accounts, Gmail automatically picks the right “from” email address when you send a reply. The trouble is that the mobile Gmail web app has a bug that reverts to the main “from” address after 10-20 seconds or so. You start typing your messages and the “from” address changes. Even if you change it back, it will revert to the main address after a few seconds. This also happens when you compose a new message, not just when you send a reply.