"There are no unintended consequences--only unwanted consequences." Slim Fairview
If you have assembled a team that includes people with experience, you can minimize the unknown unknowns.
Perhaps what you mean are the unforseeables that can't be foreseen. Don't let what may happen complicate the task at hand. Too many people, (Me included) have worried about what might happen that they create problems for them selves that would not have happened it they'd been doing their jobs in the first place instead of worrying. (There is a sentence in there, somewhere, some assembly required.)
Needless to say, when the problem comes up, they declare it the result of an unknown unknown. Don't worry about it.
Assemble a team with people who have experience. (In different areas.) Keep a network of people who've had to work in crisis management situations. (It needn't have been a big crisis, the thought process is the same.)
Then get back to work. If something should crop up, (dare I say it) call a meeting.
A Small Meeting. One with a few people skilled in the particular area of contention.
Now, it should be just about tea time across the pond, enjoy a glass of chateau Fleet Street, and remember the admonition of Horace Rumpole: Never ask the witness a question unless you yourself know the answer.