Randomize usr 1302
Well I never knew that! I knew about 1331 and the format because I spent a happy week writing my own turbo loader for laughs but never discovered 1302.
On a related note I've always credited The Sinclar Spectrum ROM Dissasembly (PDF) as one of my landmark reads. I will always remember the moment that I realised why you could use the most significant bit of a mantissa to hold the sign flag. A little light bulb glowed bright in my little head back then :)
That and the eponymous Zacks bible Programming the Z80 (also PDF) between them kindled my interest in programming. Not so much 'doing something clever' as that involved a lot of work. More the idea of 'being the perfect servant in the back room'.