Long notes cut short and not counted. Seriously though, if you make a mistake, keep going. Don't spoil the flow. Stopping draws attention to it. Of course if it's a total 'shipwreck' and it cannot be salvaged it would be better not to continue, but as a general rule, carry on.
Mental blanks seem to be the most common problem.
Give yourself time. Take a few seconds before you start and have a quick mental run through the scale in question. Do not attempt to play it faster than you are capable. It's better to play it slower and correctly than fast and completely wrong. Common faults - Launching into the wrong scale without thinking.