Loops
Defects in the structure and control of iteration, where the loop’s own machinery — its counter, bound, or update — is malformed. The classic case is a loop whose controlling variable is missing or never advanced toward its termination condition, so the iteration either fails to progress as intended or runs without the bound the code assumed.
Such mistakes are easy to overlook because the loop body may look complete and correct; the fault lies in the scaffolding around it, which determines whether the loop runs the right number of times, the wrong number, or forever.