"unforeseen problem" is a correct and usable part of sentence in written English. You can use it to refer to a difficulty or issue that was not expected. For example: "The project encountered an unforeseen problem which caused a delay in completion.".
Instead, she awoke from surgery with an unforeseen problem.
But then an unforeseen problem emerges: the name of the designer is revealed as Mohammad Khan.
Vann then flew back to Moc Hoa to try to resolve an unforeseen problem in refuelling the helicopters.
The plant had no safety systems designed to cope with a vessel head failure, also an unforeseen problem.
But they also present some unforeseen problems.
So what "unforeseen problems" is Professor Sharkey referring to?
In some pilot projects, there have been unforeseen problems.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK