Hi,
I came across this glitch while I was vertex painting my floor.
My grid in the editor changed into these X and Y planes.
It persists if I open a different level but in the same project.
Turn your Gridsnapping to something lower you have it at 10.000 units 
Thank you! ^^
Good to know, must have made a misclick there.
Happens to the Best =) one accidental click or Hotkey Combination and then the Fun begins to figure out what you triggered xD