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That's the point that you inform them that they brought you in for your technical expertise and that you have advised them of the difficulty of the task. Remind them that, when you overrun other tasks because this one has been underestimated, that you will be referring back to the meeting as to the reason why other tasks have not been completed. Send a confirmation email afterwards otherwise they will "forget" that it was their idea.
In my own career, one of my favorites was being tasked with designing a single server application with "unlimited capacity". I patiently tried to explain that its not possible to have infinite capacity in the real world...that some resource would always limit capacity.
The response, "You're being unreasonable, I said 'unlimited' not 'infinite'".
I simply sighed and resisted the urge to fetch a dictionary. Days like these are why they invented beer
Thank you so much for confirming that I'm not the only one! I feel better now!
A similar story...
Two weeks ago my non-technical colleague got a customer request for a special export...should be simple...only it wasn't...and around 60 hours later, and it's finally done except for the documentation. My colleague actually told me not to waste time on documentation 'cause nobody will read it anyway, her included. That's OK, the UI is self-documenting! But she's right...in my quest to turn a one-off into something truly useful, I created a screen that only a developer I can understand or attempt to use. (or job security?)