A mask is meant to help a superhero (or heroine) conceal their secret identity from the world, to protect them and their loved ones from the vengeful enemies seeking payback or to strike a personal blow. Long before Spider-Man revealed his identity as Peter Parker in Civil War, his teenage incarnation in Ultimate Spider-Man was the textbook example of what happens when you can't keep your damn mask on. Besides all the Ultimate superheroes, SHIELD agents, and civilians who know Peter Parker is Spider-Man, Peter's identity is also known to many of his primary villains.
The first to figure it out was Norman Osborn, who realized his OZ-infected spider gave Peter his superpowers, and his attempt to replicate that success onto himself caused Norman to transform into the Green Goblin, sparking the first of their many conflicts against each other. This knowledge was passed onto Doctor Otto Octavius, Norman's former employee now better known as Doctor Octopus, who put two and two together while imprisoned by SHIELD following his defeat by Spider-Man. Peter's former friend Eddie Brock, Jr was the third to learn of Peter's identity and later confronted Spider-Man as Venom, justifying his turn to villainy due to Peter's attempt to destroy his father's work and not telling him about his other life. When Norman and Otto formed the Sinister Six, they kidnapped an unmasked Peter and revealed his true identity to Electro, Sandman, and Kraven the Hunter. The super villains blackmailed Peter into joining them in an attack on the White House, where the Six lost to the Ultimates, but Peter's former best friend Harry Osborn would also learn the truth about Peter.
In the "Carnage" story-line, Peter sought medical help from Doctor Curt Conners, whom Peter had helped when Curt had transformed into the Lizard after they fought in Ultimate Marvel Team-Up. Since Curt created the organism deemed Ultimate Carnage from Peter's blood sample, one could say Carnage's appearance at the Parker household would prove it remembered Peter's secret from his memories. Harry Osborn would return, remembering Peter's identity, and reveal that he, too, had been affected by the accident that created his father and Octavius' powers, turning into the Ultimate Hobgoblin.
In the 'Tainted Love' story-line, the shape-changing Chameleon and his sister Camellia learned of Peter's secret identity when they kidnapped and impersonated Spider-Man, wreaking havoc on his personal and professional life. The last major villain to learn of Peter's identity was Blackie Drago, aka the Vulture, who joined the other Spidey villains to kill the Wall Crawler under Norman Osborn's leadership. Taking down the Vulture with ease, Spider-Man (with the help of some friends) was able to defeat them all.
While this most likely was not the legacy Peter Parker had in mind, this running joke has now bled into the MCU, where the young Peter Parker has trouble keeping his identity secret from other superheroes, enemies and even classmates. He was even famously publicly outed by Quentin Beck's Mysterio at the end of Spider-Man: Far From Home. While the idea of a secret identity today seems ridiculous considering surveillance and how fast information travels, one would think that your friendly neighborhood Spider-Man would have started exercising some more caution.