A feature that someone, who is not part of the actual implementation, demands for the wrong reasons. For example, because of a desire to follow some current trend, political maneuvering, an unhealthy fascination with technology, or maybe infatuation with his/her intelligence.
A feature with unrealistic requirements. Low-cost, high-performance, with perfect user experience, bug-free, delivered yesterday.
A feature that a member of the project team wants to implement – or even worse – has already implemented. The emotional attachment makes it hard to abandon the “baby”, even if it would be better for the overall project.