"Unforeseen challenges" refers to difficulties or problems that arise unexpectedly, which we did not anticipate or could not have predicted beforehand. Full definition
Plenty of unforeseen challenges arose when I made my switch, yet, despite the steep learning curve, I can't imagine ever returning to a «regular» house. (mindbodygreen.com)
From beginning to end, the adoption process can be overwhelming and bring unforeseen challenges. (mareinc.org)
With coaching, school leadership teams developed contingency plans so that they could provide sufficient time for productive collaboration even when unforeseen challenges emerged. (leadingeducators.org)