One that continuously comes up at places I work is email validation. Every place I've worked there's someone who want to ensure that users providing "real, valid emails", and no matter how often or clearly I explain why that's a hard problem, I've always seen the team tasked with putting something to attempt it. These days I mostly end up giving the "if you use that ad-hoc validation scheme, you'll block x% of potential signups", and let the business decide if x% is acceptable. That, or I tell them to integrate 3rd party accounts and let people user their google, facebook, or whatever login.