Bother!

Can you spot the deliberate mistake?

Mary and I went to Harborough Rocks - nice and near, short and easy, what could be better?  
I should've known that there would still be a lot of snow there, but I didn't.
I should've been prepared for the groups of children there, but I wasn't.
I should've checked my shoes, but I hadn't.
So, what with the two right feet, the snow and ice and the cold....
We went for a walk instead :-)