Slightly But paint over a vent hole can have serious consequencies, HM Submarine Thetis. The yard had painted over an inspection hole in one of the inner bow torpedo tube doors. A crew member opened the inner door thinking the tube was empty. Thetis subsequently sank in Liverpool bay whilst on her sea trials.
Lieutenant Frederick Woods, the torpedo officer, opened the test cocks on the tubes. Unfortunately, the test cock on tube number 5 was blocked by some enamel paint so no water flowed out even though the bow cap was open. Prickers to clear the test cocks had been provided but they were not used. This combined with a confusing layout of the bow cap indicators — they were arranged in a vertical line with 5 at the bottom - 1,2,3,4,6, and then 5; and the shut position for tube 5 on the dial was in a different position from those of the other torpedo tubes — led to the inner door of the tube being opened. The inrush of water caused the bow of the submarine to sink to the seabed 150 ft (46 m) below the surface.
http://en.wikipedia.org/wiki/HMS_Thetis_(N25)