If there is one all encompassing rule in the Galactic Federation, one that EVERY officer of EVERY ship must follow to the letter, it's the prime directive.
First introduced way back in the original series, the prime directive is very simple: if a planet's dominant species has not yet achieved warp technology levels, then Federation officers are not to speak with, interact with, or share higher level tech with them if they can at all help it. It's a good rule on paper, since it basically forbids officers from landing on a random primitive planet and declaring themselves a god.
However, that doesn't stop them from doing that anyway. A lot.
The prime directive sounds great on paper, but its apparently harsh punishments for breaking it haven't stopped a LOT of characters throughout the show's history breaking this rule with reckless abandon. To the point where it's a cliche of the show that the captain will solemnly look into the middle distance and declare that they're going to break the prime directive. Which honestly loses its gravitas after a while.
To prove this, we're going to look at the 10 most notable times this happened. Whether notable for how justified, unjustified, or just plain freaking SILLY the reason is, these are the ten most memorable moments where Star Trek broke the prime directive.