You’re thinking of the Underhanded C contest: http://www.underhanded-c.org/. The goal is to write C code which looks completely innocent, but which actually performs some malicious or unwanted behaviour.

In the last year they ran the contest (2016), the goal was to write a program that would compare two sets of measurements for similarity (nominally radiation emission spectra) but fail to give a correct answer when given a particular crafted input.