I have done this so many times that I don't even use Flash anymore. I'll be running to a wall or standing near one and I'll just flash into the wall and not go over it.
What makes this happen? Is there a target distance on Flash like Corki's Valkyrie or does Flash just not work sometimes? In the description it says it moves target champion towards the direction of the mouse, but why can't I get over the wall half the time?