@a and @e selectors can be used in commands such as /w, /me /say etc to flood chat due to the fact that when inserted as an argument to the command, it causes the selectors to make an output. 

If this is done continuously, it can cause clients on the server to crash. 

Examples:

/me @e @e @e

/w @a @e

Both of these will output a list of all entries in the world in chat causing spam. This can be abused by players to cause unwanted spam.