On the 300, I had to ferry the plane from a secondary airport to the main one during the night after a I had a 2 sector commercial flight.The plane completed some tech work and released for flight few minutes before I arrived to pick it up. I did the walk around and nothing has been noticed. The flight was really short, the airports being 6 NM apart, and planed to be flown at 3000 ft with flaps 5. As I was PF (from LHS) I have started to level off at 3000 ft when the F/O called out the altitude mentioning that I was climbing thru 3000 ft and speed was increasing. According to my instruments everything was fine but after a quick cross check with stby instruments we realized that the F/O indications went crazy. I called for recall items “unreliable speed” followed by the QRH but it meant nothing more than deciding that my instruments were correct, no icing present and we carried out the app and landing without any event.
After landing and snag reported the technicians discovered a white plastic ring on the F/O pitot tube. Later we learned that they were using 737 NG pitot covers on the 300. The 300’s pitot tube is larger and has specific shape and hence a specific cover. The NG cover has a plastic ring to sty fit with the tube and somehow, on that night, when the technicians removed the pitot tubes covers they tookoff the cover but the ring remained stuck there clogging the small static holes.