Today, I submitted a Python3 program to the "Going Home" Problem in Contest 707. My program passed all pretests, but apparenty had a timeout when tested. After the contest I looked at some other Python3 solutions, and they looked very similar to my solution. I changed a few minor things (every version was accepted). Eventually I decided to submit my original contest solution again. To my surprise it now passed. Barely, with 1996ms, but it passed.

Lesson learned:

1) Even if pretests are passed, it's probably safer to submit with PyPy instead of Python. 2) When your time is close to the time limit, you may be lucky, or you may be unlucky.