What an odd question.
I have found that what is "unexpected" depends greatly on the knowledge and experience of the operator and the engineering company you are buddied up with.
The other variable is the dilligence, or otherwise, of the people previously operating and/or maintaining the aircraft.
We bought 2 aircraft from an RPT operator 18 months ago and for reasons related to the factors described above, our learning process has been long and, on occasion, expensive.
It has also meant that what is "unexpected" is now a very small range of possibilities, because frankly most of what previously was "unexpected" has now happened or been narrowly averted and is "unexpected" no longer.
There is still the possibility of a vast number of things that could unexpectedly go wrong... but I am not expecting them to.
...as one would expect.
