Just catching up on the latest series and I'm eternally amazed what some criminals will do.  One chap committed a crime at his workplace one night then turned up the next morning and strolled past the police who were guarding the crime scene only to be arrested moments later when plod noticed he matched the description of the suspect.  I can only assume that some how he thought he'd be able to just claim his DNA was at the scene because he'd been working there.