Yep. Then it's either extra billable hours for you to fix the mistake you knew was going to happen, and/or the lawyers get involved and point out the mistake being requested in whatever statement of work. Even better if you called out the exact failure scenario in the risks section that you suspect they didn't read.
At the end of the day, it doesn't really matter if they listen/understand or not, as long as you documented telling them or it makes it into some legal document and they still pay the bill.
If you offer a client an option with a 1% chance of a very bad outcome and they understand and accept the risk, sure.
But if they have not understood and accepted the risk? Implementing something you already know the client misunderstands ain't always a smart move.