Socrates' death sentence was more or less, a mercy killing. Plato's Apology, which is the narrative of Socrates trial, seems to suggest that the Athenian Assembly knew putting Socrates to death was unjust. They simply wanted a scape goat to blame the "Thirty Tyrants" on, and make a political message against Oligarchies. The ideal outcome would have been to have Socrates admit the faults in Sophists and radical teachings. Sophists were teachers for young pupils, often leaning on oligarchic principles but they were hired to mature young men (this included intimate relationships. It's important to note this wasn't what people were outraged about, in fact this was an accepted/encouraged practice. The criticisms centered on Sophist teachings being counter-intuitive to pure democracy). Plato suggests that the Assembly did not want to kill Socrates. They actually plea-bargained and offered relatively lax punishments (will look for exact examples when I'm home from work). However, in this case, it's not exactly Socrates' stubbornness... I suppose his strong reserve would better describe why he willfully chose to reject any deal the Assembly proposed, and thus accepted a death sentence. Athens executions were usually stoning, or a public death by javelin, but the hemlock was a merciful execution so he can enjoy his final moment with his friends.

I don't have any other examples of a death by hemlock imbibe, but I wanted to put into context why Socrates was given the hemlock as opposed to receiving the usual methods of execution (and why the guards turned a blind eye).

Not sure if this really counts, but my only sources are Plato's Apology and our analysis in my 4000 level Ancient Greece class. I do apologize if that doesn't qualify.