Many People seem to have the same misconception you've perpetuated here, that a reactor can be SCRAM'd just by dropping the rods into it. The fact of the matter is that a reactor has a MASSIVE latent heat of reaction (this doesn't tend to happen as much in military reactors because they are near weapons grade and thus have less radioactive by-products that need to decay). This heat MUST be dissipated or the core will melt. One way to get around this issue is to use a natural circulation reactor. Or to maintain an extra supply of coolent on a gravity feed.
If it happens once, it's a bug. If it happens twice, it's a feature. If it happens more than twice, it's a design philosophy.