This is a mild one but I still think it's a little weird so here we go. Also English isn't my first language so forgive me for weird grammar and typos.
I normally DM a Pathfinder 1e game but because of scheduling we only play once a month at max. That's why I decided to look for games near me and I found a 5e game I could join.
It was an ongoing homebrew campaign, all PC's where level 10 which was quite challenging for me because I really don't know anything about 5e but I managed to roll up a character.
The DM provided me with the lore of the world and said you'd get bonuses if you worship the gods. There were four gods and I picked the one who was considered evil on the surface but if you actually read the lore, you knew it was the good guy.
Enough prep, I entered the game and told everyone out of character what I was playing. Immediately after I met the party in game one player asked me what God I'm worshipping. That's weird first question to ask but knowing my god was considered evil in this world I lied and rolled a natural 20 which came out to a 23 or something. They rolled insight and got a non natural 27.
I thought I was safe but that's when I learned that a natural 20 doesn't matter on a skill check so the player said they knew who I was worshipping and wanted to fight me. However the DM threw me a bone and said: "You only know he's lying, you don't know what truth he's actually hiding."
They were very upset but asked to roll to persuade me to telling the truth. Again I thought I was safe because that kind of roll isn't a thing on my usual table but the DM allowed it. The player rolled a 28 or something like that and the DM asked me for an opposing roll. I knew that was pointless as even a natural 20 wouldn't get me there but I rolled anyways and got a 5.
Well that resulted in me saying hello, being recognized as a follower of an evil God and me telling them all about it within the first interaction.
You can imagine that integrating my character into the party was pretty difficult at that point but we kind of handwaved it I guess and the other player was satisfied knowing my secret and being able to look down on him.
I continued to play but I was getting a bad vibe from this interaction and I was proven right in the future.
TLDR: I learned the hard way about skill checks in DND and another player metagamed to expose my character.