While it made some of the public distrust him initially, Spider-Man's full-face mask served multiple purposes when he first designed his suit, as he was trying to conceal both his age and his identity to keep his loved ones safe. However, he hasn't had the best luck with keeping his mask on during his career, putting Peter Parker's secret identity at risk.
Without counting the times his identity was discovered or shared without actually unmasking to characters like Daredevil, Wolverine, and Venom who were able to sense or smell him, Spider-Man has still had his mask removed multiple times over the years by himself or others, which makes it surprising that his secret identity hasn't leaked to the public more often.
10 Doctor Octopus Easily Defeated And Unmasked Spider-Man - Amazing Spider-Man #12 (1964)
Spider-Man struggled during his first encounter with Doctor Octopus, and when they met for the second time in 1964's Amazing Spider-Man #12 by Stan Lee and Steve Ditko, he was also dealing with a cold that made his spider-powers vanish, a recurring problem in his early days.
Unfortunately, Doctor Octopus had kidnapped Betty Brant so Spider-Man was forced into action. Doctor Octopus easily defeated Spider-Man and unmasked him in front of the crowd, though they assumed Peter had only posed as Spider-Man to save Betty, and his identity remained hidden.
9 Green Goblin Used A Spider-Sense Dulling Gas To Spy On Spider-Man - Amazing Spider-Man #39 (1966)
Spider-Man and Green Goblin's early relationship was restricted to their costumed alter egos, though things changed dramatically after "How Green Was My Goblin!" by Stan Lee and John Romita, Sr. in 1966's Amazing Spider-Man #39.
Green Goblin had developed a gas that was able to dampen Spider-Man's spider-sense, which allowed him to follow the wall-crawler and catch him removing his mask. This led to Green Goblin's own unmasking as Norman Osborn, making their ongoing battle personal for the first, but definitely not the last time.
8 Peter Parker Unmasked To His Friends While Sick - Amazing Spider-Man #87 (1970)
The flu reared it's ugly head in Peter Parker's life again in 1970's Amazing Spider-Man #87 by Stan Lee and John Romita, Sr. and true to form for classic Spider-Man, he began to lose his powers at crucial times. The flu also began to affect his judgment, so when his powers disappeared he decided to reveal his secret identity to his closest friends at Gwen Stacy's birthday party.
He strolled in with his mask in hand and revealed his identity, though he quickly rushed off and finally saw a doctor. When he began to feel better he realized what he had done, so he was forced to ask Hobie Brown/Prowler to impersonate Spider-Man so he could cast doubt on his previous unmasking.
7 Spencer Smythe's Spider-Slayer Caught Spider-Man Changing - Amazing Spider-Man #105 (1972)
J. Jonah Jameson's early grudge against Spider-Man led him to take drastic measures that included transforming villains like the Scorpion and hiring scientists like Spencer Smythe to create robotic Spider-Slayers.
1972's Amazing Spider-Man #105 by Stan Lee and Gil Kane introduced the Mark III Spider-Slayer, which was able to follow Spider-Man undetected and catch Spider-Man as he changed out of his costume. When Spider-Man realized what had happened, he had a rubber Peter Parker mask created so he could fool the Spider-Slayer the next time it appeared.
6 Jameson Revealed Photos Of Spider-Man Disposing Of His Unmasked Clone - Amazing Spider-Man #169 (1977)
After Spider-Man's fatal clash with his clone during the original Clone Saga, he was forced to dispose of the body in a smokestack, though his spider-sense warned him too late and he was caught on camera with the unmasked clone. Jameson finally revealed to Peter Parker that he had the photos in 1977's Amazing Spider-Man #169 by Len Wein and Ross Andru.
Thankfully, Peter had discovered the photos in Jameson's office previously, which gave him time to create fake photos that made the real photos appear to be doctored, saving his identity and earning a rare apology from Jameson.
5 Spider-Man Revealed His Identity To The Burglar Who Killed Uncle Ben - Amazing Spider-Man #200 (1980)
After Peter Parker's childhood home was rented out and trashed, Spider-Man was shocked to discover in 1980's Amazing Spider-Man #200 by Marv Wolfman and Keith Pollard that the person responsible was the very same burglar who had shot and killed his Uncle Ben.
It was revealed that the burglar had been searching for a treasure hidden within the house, which brought him into a confrontation with Peter Parker as well. When Spider-Man later cornered the burglar, he revealed his identity as Ben Parker's nephew, surprising the burglar and triggering a fatal heart attack.
4 Peter Parker Unmasked For "The Kid Who Collected Spider-Man" - Amazing Spider-Man #248 (1984)
Roger Stern and Ron Frenz brought readers the iconic back-up story "The Kid Who Collected Spider-Man" in 1984's Amazing Spider-Man #248, which featured a unique meeting as Spider-Man dropped in for a visit with a young fan named Timmy. Spider-Man and Timmy took a walk down memory lane as he shared with his hero the many clippings he had taken of Spider-Man.
Spider-Man than surprisingly shared his origin with young Timmy, revealing the loss of his Uncle Ben and his motivations to this apparent stranger. Spider-Man then even more surprisingly unmasked and shared his identity as Peter Parker upon Timmy's request, though as Spider-Man left it was revealed that Timmy was sick with leukemia and only had weeks to live.
3 Aunt May Finally Discovered Her Nephew Was Spider-Man - Amazing Spider-Man #35 (2001)
Following a heated battle with the powerful Morlun, a battered and exhausted Peter Parker managed to make it home to his apartment where he discarded his tattered costume on the floor and passed out in bed.
Unfortunately, he had neglected to call his worried Aunt May, who showed up at his apartment while he was sleeping and discovered his costume on the floor. May kept the revelation secret until a few issues later in J. Michael Straczynski and John Romita, Jr.'s "The Conversation" brought Peter's secret finally out in the open.
2 Peter Parker Unmasked In Front Of The Whole World - Civil War #2 (2006)
Everything changed for Spider-Man and the rest of the Marvel universe of characters during Mark Millar and Steve McNiven's Civil War event in 2006, which forced superheroes to register with the government following a tragic disaster.
Spider-Man had formed a tight bond with Tony Stark/Iron Man who was in support of registration, and he convinced Spider-Man to unmask at a press conference to make a statement. Unfortunately, the unmasking led to the near-death of Aunt May and Spider-Man's deal with Mephisto in "One More Day" that rewrote Spider-Man's history.
1 Spider-Man Revealed His Identity To J. Jonah Jameson - Peter Parker: The Spectacular Spider-Man #6 (2018)
After Spider-Man erased the knowledge of his secret identity from the world using a mystical spell, he unmasked a few times over the years to people like the Fantastic Four and the Avengers who had already discovered his identity in the past.
However, Peter Parker's next big unmasking occurred in the pages of Peter Parker: The Spectacular Spider-Man #6 by Chip Zdarsky and Michael Walsh when he revealed his identity to longtime rival J. Jonah Jameson during a heated interview, leading the characters to move forward in their relationship for the first time as allies.