As an undergrad doing an internship helping some PhD students write a complex MATLAB simulation. Part of the simulation needed to be done in a loop and, naturally, I used "i" as the index variable. I spent several days debugging why the expected output result was not correct.

Turns out there were some calculations using complex numbers (e.g. `x = 2 + 3*i`) and MATLAB decided that I obviously wanted to redefine the imaginary number constant in the loop. It was not very funny at the time.