Command block selectors not working

Occasionally, command blocks using player selectors such as @p, @a, @r, or @e will perform the command on the literal player "@p[...]" instead of searching for a player with the specified parameters. For example, the command "effect @p 16 999999" will give a player named "@p" effect 16. if the parameters in the @p selector exceed 16 characters, the game throws an exception, seemingly because the "name" stored in the scoreboard is longer than the allowable length of a player name.