When bad programming (or a bad compiler/loader) causes a computer to 'write' data over a portion of memory that it later thinks is programming instructions in machine code, it will end up doing unpredictable things, depending on that data. By extension, any action which is likely to have unpredictable, unforseen or chaotic results.
Usually heard among techies who program in assembly language or machine code.
"I wouldn't tell your wife about your one-night stand in Tulsa if I were you; that would be self-modifying code."
To change an object so as to customize its use to fit your needs, such as taking out a set of P90 pickups on a guitar and replacing them with Mini Humbuckers.
Jim: "Bruh why's your car so loud?"
Bob: "I had to modify it to sound badass!"
When some resigns or retires from a job, they are on "eternity leave". This can also refer to someone who has passed away, especially someone who worked somewhere for a long time, retired/resigned, then passed away later.
Oh I'm sorry, you didn't know that Kelly is on eternity leave? She resigned and moved to Hawaii!
With regrets I would like to inform you that my boss is officially on eternity leave. I will never see or hear them again. Ever.