How common were World War II bomb duds?

How often were World War II bombs duds, as happened during the Falklands War?

More often than is commonly admitted, because release height, fuze faults, and impact angle could all prevent detonation.

World War II bombs also failed more often than many readers expect, much as weapons did during the Falklands War. A weapon released too low might strike before its fuze armed. Faulty detonators and unfavorable impact angles created additional failures. Royal Navy bomb-disposal histories and unexploded weapons still found across Europe preserve evidence of that less visible side of air attack.