Me, whenever I compliment someone.

This troper falls into this trap a lot. I tend to be very sarcastic, so whenever I actually say something nice, people won't believe me. "I'm in SincerityMode" will probably be uttered a lot from now on.

Sometimes I have trouble with this, but my eyes are quite expressive, so generally I just need to look the person in the eye for them to know I'm being serious.

Tropers/{{Excel-2010}}. I'm rarely completely serious about anything, but those wrongfully arrested or convicted always have my full sympathy. I never joke about those who serve time they really shouldn't have, because no amount of compensation can restore lost time with their families and friends.

It's usually hard to get a serious conversation out of my LoveInterest in RealLife, but he's completely honest when talking to my ShipperOnDeck best friend.

Tropers/ManCalledTrue codeword for "Sincerity Mode": "This is a serious question and I want a serious answer". Makes my {{Cloudcuckoolander}} friend immediately straighten out.

This troper used to say "yeah, right" as an affirmative response to almost anything when he was in kindergarten or so. It's too bad he had no idea it also meant a sarcastic response.

This troper gets into this kind of situation often with my mother. Because of my history of condescending sarcasm, she often assumes I'm being a jerk when I mean something genuinely, especially with my older brother. For example: after correcting a mistaken belief of his, I might ask something along the lines of "Why did you believe that?", which my mom will interpret as "You're really stupid for believing that," when in reality, I just wanted to know how he came to hold that mistaken belief. I try to change my inflection so it doesn't sound sarcastic, but there are just some things that will sound sarcastic no matter how you say them.