This classic police blunder is taken from the autobiography of Keith Hellawell, who is prominent in the UK:
One case nearly backfired on the officers involved. A man had been found dead in a one-up, one-down house on the outskirts of town. He had sustained multiple injuries, which the detectives managed to convince the Coroner's Court were self-inflicted. This was no mean feat, as he had been strangled, hit over the head, stabbed several times and then gassed.
My colleagues claimed to have deduced that he was desperate to kill himself and had tried to strangle himself with the flex from the hanging light-bulb at the top of the stairs. They insisted his weight tore the fitting from the ceiling - a task undertaken, in fact, by the investigating officer - and he fell down the stairs, sustaining his head injuries.
He survived the fall, and so, still determined, he crawled to a drawer, from which he pulled out a knife and stabbed himself. Unfortunately the knife had mysteriously disappeared from the scene, so one had to be provided for authenticity.
In the end, still alive, he put his head in the oven and turned on the gas, by which means he finally achieved his task.
There was a great deal of embarrassment when a man gave himself up and confessed to the murder. He served life imprisonment.
(From 'The Outsider' by Keith Hellawell, HarperCollins.)