People are different. Some freeze up. I get ultra focused. I tend to zone-out everyone and everything except the task at hand. That is how I know I'm pushing myself.
Some people fall apart.
Not a good thing. They make mistakes. You usually don't realize it until you've crossed the line. Actually, you never really know you're at your limits until it's much too late to do anything about it. What is important, however, is how you deal with it.