Mojave: Pasting text with control characters
Update your OS to 10.14.1 or later

It seems that Terminal in 10.14 would present this dialog when pasting a tab character, but the updated Terminal in 10.14.1 and 10.14.2 does not.

I was scratching my head as to why I was only able to reproduce this issue on one install but not another. Unable to find any settings that had any impact, I eventually tried updating the machine that showed the dialog, and it was gone.