system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:If nurses have to be so careful not to oversedate patients, why is it so hard for executioners to overdose a prisoner?

What I mean is, why are there so many botched executions? Couldn't be the vein every time, could it?
gpt
A conventional lethal injection protocol consists of thiopentone, pancuronium and potassium chloride. Either of those is pretty much guaranteed to be lethal on its own, as long as it is given intravenously.  

Unfortunately, both thiopentone and potassium chloride are extremely irritant and painful when injected into subcutaneous tissue.  

They get into trouble in executions because executioners are terrible at obtaining intravenous access. They inject these drugs into an incorrectly placed cannula, so instead of killing the prisoner they give him an excruciatingly painful burn around the injection site.