Mistaken for Cheating/Playing With

Basic Trope: A character believes his/her significant other is cheating on them.
 * Straight: Alice sees her boyfriend Bob and his friend Claire alone together and assumes he's cheating.
 * Exaggerated: Bob strikes up an innocuous conversation with Claire, which arouses Alice's suspicions.
 * Justified: Alice's previous boyfriend cheated on her, so she's more likely to assume other guys are cheating.
 * Or Bob has a known history of cheating of his girlfriends, but is working hard to amend his ways.
 * Inverted: Bob and Claire make out in front of Alice - and she doesn't suspect a thing.
 * Subverted: It turns out Bob really was cheating on Alice.
 * Double Subverted: With her clone.
 * Parodied: Alice, of all people, turns out to be the other woman.
 * Deconstructed: Alice's constant suspicions start to derail the relationship and end up destroying it, as the constantly-faithful Bob decides he's had enough of it.
 * Reconstructed: Claire is eventually able to prove Bob's innocence, leading Claire to try to win Bob back over and repair their relationship.
 * Zig Zagged: However, it turns out Bob really WAS cheating on her, with Alice's Sister, who asked Claire to tell Alice nothing happened between Claire and Bob.
 * Averted: Alice doesn't assume Bob is cheating.
 * Enforced: "Alice and Bob are happy, right? We can't have that!"
 * Lampshaded: "I hope Alice doesn't think there's anything between us!"
 * Invoked: When Bob believes Alice is losing interest in him, he starts hanging around with Claire to make her jealous.
 * Defied: Bob refuses to even do anything that could be construed as romantic with any other females, because he doesn't want to make Alice assume anything.
 * Discussed: "It's all a mistake! I wasn't cheating!"
 * Conversed: "Oh, wow, another plot where she thinks he's cheating on her."

Honey, I can explain the Lipstick Mark, the bra in my bed, and the photos! Just go back to Mistaken for Cheating!