I took a Computer Architecture class for undergrad, and one of the pieces of coursework was to implement a Karatsuba's Algorithm in raw UAL Arm Assembly, using some weird non-standard (almost Chen-Ho, but not quite) decimal encoding of numbers devised by the instructor to "make things interesting". The idea was for it to support arbitrarily large numbers encoded using this format, as opposed to the considerably more trivial requirement that it just support stdint style integers.

It was considerably harder than it sounded (very few people in the class actually managed to get something working, since it was 'optional' but could still raise your grade), and I had daily nightmares about stack frame corruption throughout the project.