Bitmasks
Defects in code that uses bitmasks to pack, test, or clear individual flags, where the mask itself is wrong for the value it is applied to. The typical fault is a mask whose set bits do not line up with the field being manipulated — a stale constant, a wrong width, or an off-by-one shift — so the operation reads or modifies bits the author never intended.
Because a masked &, |, or &~ is always a legal expression, the compiler
offers no warning; the result is a silently incorrect flag set that misroutes
logic or corrupts a packed structure.