I found this by code inspection. The linenoise.cpp file handles line editing in the mongo shell and it emulates some bash & emacs keystrokes such as ctrl-b to go back a character, ctrl-e to go to end-of-line and ctrl-d to delete the character under the cursor. ctrl-d works fine on most lines, but behaves differently on lines with zero or one characters on them. On a zero length line, it exits the program. This is deliberate and the bash shell does the same thing. On a one character line when the cursor is positioned over the single character, ctrl-d does nothing. The code has always been this way ... the code at git://github.com/antirez/linenoise.git is identical in this section. The Delete key is similar to ctrl-d but works properly and doesn't exit the program if pressed on an empty line. It will delete a single character.