Cruel and Unusual Punishment

The Eighth Amendment protects against "cruel and unusual punishments."  The amendment is open to high controversy due to the ambiguity of the term.  What can, lawfully, be considered cruel and unusual?

It's often agreeable that things such as executions should be as painless as possible to avoid infringing on the Eighth Amendment.  The electric chair, previously popular as an execution device, has been in decline in the past few decades.  The electric shocks occasionally didn't kill the subject outright, leaving them in an incredibly pained condition before dying (this is akin to the medieval practice of bribing an executioner to sharpen his axe, so that the execution doesn't take more than one stroke).  A very unique case over the electric chair was Louisiana ex rel. Francis v. Resweber,
in which a man who was subject to execution but did not die due to equipment failure sued that a second attempt would go against the double jeopardy clause.  His case constituted that since he was "executed," a second execution would be as if he was tried twice for the same crime.  It was noted at the trial that given an infinite number of failed executions, one would eventually cross a threshold where it becomes cruel and unusual punishment; if, theoretically, someone were electrocuted fifteen failed times, then that is fifteen times as cruel as a single execution.  Eventually, it would violate the "cruel and unusual punishment" clause of the Eighth Amendment.

The electric chair has slowly been replaced by lethal injection, which is generally considered more humane, however it has flaws of its own.  It's very difficult to judge how cruel the execution is, considering the muscles are first paralyzed and anesthetized before introducing a chemical that stops the heart.  Hence, if the execution is in fact extremely painful, the subject has no way of conveying this information given his paralyzed state.

Also interesting to note is the application of the clause with respects to torture.  The practices of Guantanamo Bay are often under attack for being inhumane, and therefore violating the clause.  However, given that torture is, by nature, cruel, the line is much more difficult to formulate.