Which K-drama Had Great Potential but Was Ruined by Poor Story Decisions?

For me its - Sisyphus the myth ( I k most of you haven't watch the show)

Potential : A time-travel thriller starring Park Shin-hye and Cho Seung-woo with an intriguing premise.

What Went Wrong : Illogical plot twists, inconsistencies in time-travel rules, and weak character arcs turned it into a mess.