New Post: Problem in thin QR Solve

Just having thought about it a little more, I think it's because for an M×N matrix:

Full QR Decomposition
Q is M×M
R is M×N

Thin QR Decomposition
Q is M×M
R is N×N (square matrix)

This means that when the solve method checks that the dimensions are the same then they won't be and that's where it throws the error.