Lightning never strikes the same place twice

What actually happened

The physics runs the opposite way from the saying. Lightning seeks the path of least resistance to ground, which means tall, pointed, conductive, well-grounded objects. Those properties do not change after a strike, so the same tower, tree, or hilltop is hit again and again. Towers are used as lightning research instruments precisely because they are reliable targets, and the CN Tower is struck around 75 times a year.

At a finer timescale, the saying is wrong within a single flash. What you see as one bolt is typically three or four separate return strokes travelling down the same ionized channel a few tens of milliseconds apart. That is what causes the flicker. The channel is already conductive, so subsequent strokes prefer it, which is the exact opposite of the myth's claim.

Roy Sullivan, a US park ranger, was struck seven times between 1942 and 1977 and survived all of them, which is the sort of anecdote that should have ended this long ago. The saying survives because it is a nice metaphor about misfortune not repeating, and metaphors do not have to be true. Standing where lightning just hit is not safety advice.