prokopetz:

Fact: In Dungeons & Dragons 5th Edition, a warlock with the Great Old One patron gains a class feature at 10th level that causes anyone who inflicts psychic damage upon them to suffer an equal amount of damage in return.

Fact: Also in D&D5E, the bard cantrip vicious mockery inflicts psychic damage.

Conclusion: It is theoretically possible for a bard to attempt to insult a Great Old One patron warlock and in the process give herself such a bad mental image that she dies.