In 1638B - Odd Swap Sort , In the contest, 146374121 was the submission in the contest. It passed the pretest, but got TLE in the system tests. After contest, I resubmitted the same solution with a minor improvement on IO. 146472941 This improvement was very minor but It passed the tests. The python fastIO template 146473139 also got AC.
One can also say pretests were not strong enough.

Conclusion — Always add the fast IO improvements in Pypy.