Oh god, the happy path fallacy. I've seen this too many times. Product manager comes up with conceptual model X, once implementation starts people it's brought

Oh god, the happy path fallacy. I've seen this too many times. Product manager comes up with conceptual model X, once implementation starts people it's brought to attention that X will only work under condition A,B,C, which together only occur for 5% of the users. Instead of reverting X, and trying out Y instead, the heavy train to finish X has already gained momentum and exceptions are added into the UI breaking the whole flow required for X to be intuitive. The sad part is that these unreasonable conditions are usually quite obvious to spot early on. I'm just making things up now but it can easily be things like requiring the user to hand over their bank log in details so that random startup can analyze their expenses, analytics that require write access to some big corporations sql database or that every user in the whole world uses Outlook 2007 and are willing to install our plugin.