Any commands run with any uppercase letters will fail. You can't do /gamemOde, for example. If you start typing /GAMem, the chat will suggest you to use /gamemode, but if you use tab to autocomplete it, GAM will stay uppercase and the command will fail.
How to reproduce
Go into any world on 17w46a
Start writing /GAMem
When a suggestion for using /gamemode appears, press tab
The command will keep GAM uppercase, and do /GAMemode
The command will then fail, because you can't use uppercase letters in commands

What I expected to happen
...was that all of /gamemode should become into lowercase when you tab-autocomplete