Unremoved Pitot Head covers
Quick clarification, as the thread was mis titled for about 12 hours, and I'm not looking to get more read in than was there.
The problem with the aircraft was that the Alternate static ports had been covered with speed tape to prevent ingress of water during a deep wash at the aircraft's home base, and it had not been removed prior to return to service. As it was alternate ports, it would not have been immediately apparent there was a problem, so the aircraft flew for 2 days with the speed tape on the ports. As the port is polished aluminium, with no paint, and speed tape is metallic colour, it wasn't easy to see.