The bug
Executing a command with incorrect syntax for multiple entities prints the error message multiple times. Expected would be that it is printed only once.
How to reproduce
Make sure there are multiple entities in your world and then use the following command
/entitydata @e→ You should get the error message multiple times
Suggested fix: test commands for valid syntax before evaluating selectors and (in case a selector returns no entities) print something if the command syntax was valid so people are not confused when a command they run doesn't return anything in chat.