A dot with two opposite corners yellow on top. Hold them at back left and front right, with the front-left corner's yellow facing left.
Algorithms
R U R' U R' F R F' U2 R' F R F'
13 moves
YOURS
Built from
R U R' U + Sledgehammer + U2 + Sledgehammer
Loose moves
R U R' U
Trigger
Sledgehammer
R' F R F'
+ U2
Trigger
Sledgehammer
R' F R F'
Relations
Your algorithm backwards
F R' F' R U2 F R' F' R U' R U' R'
solves OLL 18
That's why your scrambles are never the inverse of the algorithm: you'd be practising OLL 18 without knowing it. CubeDojo builds a random state with this case and checks it comes out.