Six or seven years ago, I worked for a large financial organization as part of a very large effort to convert server assets from physical to virtual. The consultants on site were in bed with the vendor of a terrible piece of software designed for that purpose. After a couple weeks on the job I'd had it, and sat down in between sessions of "validating" the conversion procedure, and started writing my own software for converting Linux servers. After a couple days it was working great, and they wound up adopting my software as the default method for Linux conversions.

Years later, I'm interviewing for my current job and one of the interviewers tells me he used my converter some time later and loved it. Pretty sure it's what swung the interview for me.