The bug
Typing /help and a command that's not available (either because it doesn't exist, or because its usage is not allowed, e.g. when cheats are disabled) returns An unexpected error occurred trying to execute that command, which generates an exception on the game's log. This should be a less severe "Unknown command" error message.
Note
Prior to 18w02a this would return java.util.NoSuchElementException. It now displays a nicer message as a result of fixing [MC-121662].
Linked issues
is duplicated by
testing discovered
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Isn't fixed yet. I tested in 18w02a snapshot. :/
Bentroen
Seems to be fixed in 18w03a/b. Now returns An unexpected error occurred trying to execute that command.
pokechu22
"An unexpected error occurred trying to execute that command." – that's still an issue (the command is running into a problem and if you look in the log you'd see an exception), but the message is nicer probably as a result of fixing MC-121662. At least that's what it seems like to me.
Bentroen
Ah right, that exception is now handled but it still shouldn't result in an exception. I'll update the report info.
michael
(Ignore my last comment, I didn't reproduce it right)
Additionally, this bug appears in multiplayer servers.