The Shelf

Jane was a huge fan of DIY. She spent every weekend busy with some project or other, proudly making her home exactly the way she wanted it.
She had remodelled the entire house which was more or less just a shell when she bought it.
The shelf she had put up above her bed was causing concern though.
She had measured the timber carefully and had ensured that it was level when she put it up.
The day after the shelf went up it was very obvious it wasn't level at all. In fact it was leaning substantially. She couldn't understand how it had happened, but she took down the shelf and started again.
She measured the height at both ends and laid her spirit level on the shelf. Satisfied it was correct, she mounted it to the wall once more.
She stood back and admired her work. It looked perfect.
Two hours later Jane entered the bedroom and was shocked to see that the shelf was slanting.
She was baffled. The shelf was taken down once more and she measured, checked and double-checked everything.
Once the shelf had been put back up Jane took a nap.
When she awoke the shelf was the wonkiest it had ever been. It sloped along the wall at almost 45 degrees.
At that point Jane decided she was selling the house.