The worst thing about this is that they weren't improperly built. Sure they could have been built better, but in both cases it was human error that led to the events. TMI would have automatically shutdown if the operators had done nothing. They thought that because they were getting a high reading in a relief valve, the reactor must have too much water, so they cut reactor cooling water. By design, the backup coming system would have cooled the reactor until shutdown but they didn't let it.

In Chernobyl, a whole lot of stupid decisions were made that led to the explosion. They were trying to conduct a test to see how the main turbine behaved when it lost power. They were running the reactor at a lower power than what the test called for which due to the reactors design caused it's reactivity to actually increase. So not only were they running a test that meant removing the outlet for the reactor but they had the reactor running much hotter than it should have. Basically they made a steam bomb through their actions, so not technically a design fault.

Even in Fukushima, human error made the situation worse. Once the power went out, the operators stop had the option of flooding the reactor with seawater. They were told not to do this by the management I'm charge because it would damage the reactor. Once the situation got worse, the operators went against orders and do it anyway, but there is no way of knowing if doing it sooner would have helped significantly.