We had a situation here in Australia where there wasn't an open source way to verify the outcome of Federal senate elections. The algorithm is slightly complicated, mostly because it's set up so that it's possible to count it manually as it was devised prior to computerisation of the voting system.
In 2013 this became important: in Western Australia, we had a very close senate election. There was one critical exclusion in the count where just 14 votes determined the outcome between two candidates. Obviously the accuracy of the counting software was key; it actually crashed when they were doing the count. They restarted it, but that shook the confidence of a couple of people I knew, and so I decided to write my own software to verify the count. I knocked it up in two all-nighters:
https://github.com/grahame/dividebatur
Of course, I/we were fortunate that the electoral commission was forward-thinking enough to have published the data required to fully reproduce the count.
Some open government folks later on used the existence of my software to try and get the electoral commission to release their software system under Freedom of Information laws, so that it could be verified. I was quite amused when the commission alleged there was no way I'd done it in two nights. I had; but of course, what I had was a Python implementation of the count, not a fully-fledged electoral management system like they had.
Later on in 2017-18 we had a constitutional crisis[0], as various senators were found to hold foreign citizenship and thus be ineligible to hold office. Those ineligible senators were replaced by running a count-back of the vote, with them excluded. I happened to be the only person who had a system that could work out the results ahead of the electoral commission, so I had quite an exciting few weeks providing the media with predictions on who would take over the various seats that were lost.
Now there are better and more robust systems that have followed mine, but I must say I was quite happy with this two-day hack!
[0] https://en.wikipedia.org/wiki/2017%E2%80%9318_Australian_par...