Frequently-Broken Unbreakable Vow

A character who makes The Promise, must follow the Prime Directive, or is otherwise The Fettered will be forced to break it... repeatedly. Usually they'll face a veritable parade of situations that force them to choose whether To Be Lawful or Good, Godzilla Thresholds that require heartless necessity, and otherwise constantly force them to compromise their word and/or morals to win, do what's right, survive and/or protect loved ones.

This may take a variety of forms, for example the Actual Pacifist will be turned into a Reluctant Warrior who has to kill to protect the innocent. The Captain will have the completion of their orders and the lives of Innocent Bystanders as mutually exclusive. The Watcher will be compelled off his True Neutral vague fence sitting into taking a side. On a smaller scale, any parent or lover promising to "spend time" with a loved one will be called away to duty... not that the loved one ever understands or puts up with for long.

Expect The Fettered and The Cape (trope) with a Heroic Vow to be especially prone to suffer this, though the Noble Demon may be an infrequent target. See also Oddly Common Rarity. If the story has Magically Binding Contracts involved, they'll be as binding as warm swiss cheese. Unless it isn't.

Anime and Manga

 * Roger Smith of The Big O often says that violence is a last resort in his negotiations... but then, we would never get to his Humongous Mecha if negotiations didn't break down on a regular basis, now would we?

Comic Books

 * In the Marvel Universe the Watcher repeatedly violated his oath of non-intervention.

Live Action TV

 * Star Trek: The Original Series. Captain Kirk's willingness to break the Prime Directive whenever he needed to save the Enterprise and/or a "stagnant" culture is well known. He was also a hypocrite on the issue, condemning Captain Tracy in "The Omega Glory" for doing something he had done before and would do again. Note that Mr. Spock was also guilty for not arresting Kirk each time he did it (as noted in "The Omega Glory", any Starfleet officer who doesn't take action is as guilty as the person committing the offense).
 * Subsequent captains weren't much better.
 * In an episode of Star Trek: Enterprise, Archer imagines that maybe some day, there'll be some kind of rule or directive to save captains from having to make such judgment calls. As if.
 * Oddly enough, TNG asserted that Kirk had violated the Prime Directive in circumstances TOS clearly established was not a Prime Directive violation by the interpretation held by Starfleet at the time (they would have been in TNG, but Kirk can hardly be faulted for not knowing about regulation changes decades before they happen).
 * "The Drumhead" reveals that even Picard, by far the most diplomatic Captain, nonetheless had nine seperate violations on his record.
 * It bears mentioning that, since Federation personnel are all in service to Nielsen ratings above and beyond any such regulation, violations of the rules are inevitable in the interest of entertaining television.

Web Original

 * Discussed in the Writing Excuses episode on Comedy: putting a character in a situation in which violating their principles is easy and adhering to them is painful is a good and fairly easy way to create humour or drama, but you can't keep forcing them to break their principles or they stop being principles.

Real Life

 * Many religions consider the marriage vows to be sacred right down to til' death do us part, and many believers would therefore never consider divorce due to their beliefs. It does happen quite often, though, for a vast variety of reasons.
 * "Thou shalt not kill." (Judaism & Christianity) or "Do not kill unjustly." (Islam)
 * Averted Trope in Judaism... the original hebrew translates as "Thou shalt not commit murder." Killing during war, for self-defense, to protect the life of another, or as lawful punishment for certain crimes was all considered not murder and therefore acceptable. Which makes the commandment equivalent to the Islamic version. Although the Christian bible uses the translation above, theologians in Christianity take a similar position.