V + 기 십상이다 is the short form of the Chinese maxim 十中八九 (십중팔구), which means 'eight or nine out of ten (chance)'. It is used when a negative event has an extremely high chance of happening -- the outcome is usually fairly obvious:
한겨울에 옷을 두껍게 안 입으면 감기에 걸리기 십상이에요.
In the Korean winter, if you don't wear thick clothing, you'll very likely catch a cold.
학기 동안 수업을 제대로 듣지 않으면 기말 고사에 불합격하기 십상이에요.
If, during the semester, you don't listen in class, then in the final exams you'll most likely fail.
빙판길에서 뛰면 미끄러져 넘어지기 십상이에요.
If you run on an icy road then most likely you will fall over.
도서관에 CCTV 카메라들이 설치되어 있어서 누군가 책을 훔치려고 하면 잡히기 십상이에요.
There are CCTV cameras installed in the library, so if someone tries to steal a book they will, in all likelihood, get caught.
보지 않고 길을 건너면 사고나기 십상이에요.
If you don't look when you cross the road, there's likely to be an accident.