My girlfriend's best friend never liked me. She constantly told my girlfriend that I was controlling, boring, or probably cheating.
At first, I ignored it. Then fake screenshots appeared showing me flirting with another girl. My girlfriend confronted me, and luckily I could prove they were edited.
We later discovered her best friend had created them because she felt my girlfriend was spending less time with her. Their friendship ended, but our relationship also suffered. Even when a lie is exposed, the suspicion it creates doesn't disappear instantly.