We've all been there. You're having dinner at your new girlfriend or boyfriend's house and getting on famously with her parents. But wait, what's that? It's an old family photograph on the mantelpiece that appears to feature your own grandpa in his youth.
And then it hits. You're inadvertently dating your cousin. And it's too late.
Okay, so maybe in modern Britain, with its ever-growing population and where offspring are staying in the town they were born less and less, this is a rather rare scenario.
But in Iceland, with its measly island population of just 320,000, shared ancestry between its inhabitants is far more common, leading to far higher chance of confusing familial relations.
So much so, that developers have produced an app that encourages users to "bump in the app before you bump in the bed".
Anti-virus software developer Friðrik Skúlason had earlier produced an online database to allow Icelanders to trace their heritage online, before three app developers took the information and made a specific anti-incest feature on their Book of Icelanders Android app.
The producers, from Sad Engineer Studios, told the Digital Trends website that the app works by two users bumping their phones together to instantly see if they are related.
"The 'Incest Prevention' is a fun feature that the user can enable through the options menu which will notify him with both text and sound if the one he bumps with someone who is too related to him," they said.
"We aren't sure if other countries have such interestingly interwoven bloodlines like we do, but we’re pretty sure the Icelandic genealogy database is unique in its completeness.
The developers told Digital Trends that they're unsure if the app has prevented any unlucky matches yet, though a user on Google's Play Store said that it would have been some use to him if he'd have had it last year. At least there won't be any fights over wedding guest list spaces for husband and wife.