There's nothing worse than calling a line a finding an uninvited guest has wandered onto the field. Not only do you have to recall the correct name, but you have to break the news to the overzealous player and watch them slink off sadly like Charlie Brown after failing to kick that football. Rats.
To avoid this situation you must speak with the uttermost clarity and amplification. You mustn't slur your speech; too often names like Sean and John or Brett and Bren become confused. You mustn't misspeak—say who you mean and mean whom you say. And finally, you mustn't forget the names of your players. Nothing is worse than failing to recall (or actually not knowing) a player on your team. Review the roster list before the tournament. Believe me, this can be embarrassing.
Beyond basic slips of the tongue, you must be wary of under- or over-counting. Thou shalt count to seven. No more, no less. Seven shall be the number thou shalt count, and the number of the counting shall be seven. Eight shalt thou not count; neither count thou six, excepting that thou then proceed to seven. Nine is right out. Use your fingers to keep count if you must.
The worst scenario, though, is calling yourself onto the line when you're not even an eligible to play. You've graduated, you're washed up, and no one wants to see your slow, no-defense-playing, sorry ass on the field. The look of pure disgust and pity you see in the eyes of young men not legally old enough to drink and barely smart enough to dress themselves is seared indeliably into your consciousness. Nothing you can do, nothing you can say will restore your dignity and self-respect.