Impala shell splits each command into tokens and then converts the first token to lowercase to figure out how it should execute the command.
The splitting is done by spaces only. Thus, if the user types a TAB after the SELECT, the first token after the split becomes the SELECT plus whatever comes after it.
The command is still executed through the default Impala shell method, which sends the full command text to Impala. However, the first piece after the SELECT gets converted to lowercase:
This can easily happen if the user is copying and pasting commands into impala-shell