Y2K when it's not an issue
I got asked to run a Unix internals class on a long obsolete version for a customer who was using it in an embedded application so they could do Y2K verification. Sure I'll train up you engineers if that's really what you want, but the bits you're interested in don't care what year it is, there's epoch time measuring seconds since 1970 and there's ticks_since_boot. Just please promise me you'll switch to something newer before 19th Jan 2038, coz epoch time is in 32 signed ints. This rumbled on for months till the customer eventually agreed they didn't need it.