1) Remove any display from list slot :

scoreboard objectives setdisplay @a list

2) Create a new scoreboard variable OR reset an existing variable (let's call it MYAR)

3) Display MYVAR in sidebar slot (nothing should happen) :

scoreboard objectives setdisplay @a sidebar MYVAR

4) Display MYVAR in list slot :

scoreboard objectives setdisplay @a list MYVAR

5) Press Tab. Variable MYVAR displays with a value of 0 in list and sidebar slots, but it does not count as 0 when testing score values with commands.