There's the freak ike jason noted - that you are likely going to lose no matter what. Additionally, a well fed, prosperous animal will clot much better than something from the stampeed cartoon. Sombody noted to slowly stretch the T-cord, this is sound advice. On the larger calves with large T-cords, I sometimes pull the cord well down then start the tearing with my knife with a little nick - then stretch and pull.
Anymore, I use a callicrate bander on anything over 500#. There are adds and disadds to a knife and a bander, but I think the bander is safer on the larger calves. When we've been selling 6cwts for $800, who can afford to lose one.
Don't torture yourself over losing one, just learn from it. If you learn, it wasn't a total loss, and if you torture yourself after every loss - the cow biz sucks. Learn and let go is the toughest part of cattle ranching.