By putting in an entitydata command without the datatag spams the chat with 19 "Usage: /entitydata <entity> <datatags>" messages.
How to reproduce:
1. Execute an unfinished entitydata command that specifies an specific entity(Not a player.) ex. /entitydata @e[type=Skeleton]
intended, as you execute the command on every skeleton, all skeletons reply back to you with a correct command syntax message