I had the luck to have had to solve to though problems. Like in a performance analysis in the major telecom operator in an African country, finding many flaws in the infra-structure, circular DNSes (do not ask), and to top if all, their Internet reseller selling them the double of the real bandwidth, and to top it all, them having bridging enabled to all the country due to a vendor telling them "put this line on the central router". Or when a colleague wanted to upgrade technologies, implementing filters at the CPEs after reading DOCSIS RFCs on a cable company, and seeing the infra-structure upload traffic diving to less than half. Or picking up the Linux department and even before reimplementing all the servers, optimising servers that went from 9x% CPU utilisation to 10%. Or (re)implementing the middleware for 2 cable Internet companies, in which one of them had some functionalities in Java and I reimplemented in C to see some operations that were done in 1h being done in 5 minutes.