Most of you know this already, but for those that don't: If your opponent calls a tim out before you have a chance to challenge (fumble, ect..) you won't be able challenge it.
]If you see that you can't challenge it, as soon as the teams break the huddle while you are now on defense, quickly go offsides. This will allow you to challenge the last play run. I have had a few players do this trick to me, only to look stupid when I make them waste their timeout.