They do this in some castles to trip up the invaders

Funny story: I built the staircase heading up to my loft, and spent days sourcing the perfect planks for the risers so I'd have exactly 13 steps.

Forgot to account for the loft's floorboards and beams, now I have a 13.25-step staircase.

Somehow Lowe's only has these things in pressure-treated lumber. So I made my own for my stairs in my basement using regular lumber. And then my basement flooded.