The bug

If you enter something like /gamemode  survival (notice the double space, for the bugtracker one was replaced with  ), the preview predicts an error, but the command works.

The exact error message can vary, sometimes it wants the next, sometimes the second next part. For example:

• /gamemode  survival: No error message, just four lines with grey /gamemode and red gamemode names
• /effect give  @p strength 100 100: "Invalid name or uuid at position 12: …fect give <--[HERE]" (So it apparently expects the next argument now, the selector.)
• /effect give @p  strength 100 100: "Expected integer at position 16: … give @p <--[HERE]" (So it apparently expects the second next argument now, the duration. But if I entered the duration there, the command would fail when executing.)