I believe one test, the bomb strength was underestimated or it detonated too soon. Gave everybody a good dose.
That was the Castle Bravo test in 1954. The physicist miscalculated the yield by at least half; the actual yield was 15 Mt (largest nuke in US history) instead of the estimated 4 to 8 Mt. They screwed up on how an isotope of lithium would react. A Japanese fishing got irradiated.