My list (Mac-centric just because I have historically been Mac-centric):
a) In the flat-out failure division (i.e., I’ve actually tried it and flunked), being able to make an ftp connection from a command line and download files to my local hard drive or upload from local hard drive using “get” and “put”. Damn! (Yeah, I can look it up, but still…)
b) In the “I probably still could but it’s been a long long time” division: To send out an email with a file attachment, first BinHexxing the file attachment (converts it to ASCII text with each line limited to 70 or 80 chars or some such) then uploading it to IBM 3090 Server running VM then creating the email and doing SOMETHING (??) from the command line to append the BinHex file to the email message text.
c) Same division: To embed a font in a document file so that you would not have to have that font installed to view that document properly, by doing something (hold down option key when clicking Open, maybe?) to Font/DA Mover to get it to open files other than the System file or a Font Suitcase, and then move the font into the document file.
d) Pretty sure I remember how to do it: Adjusting the system heap. If you thought memory management was pathetic in MacOS 9, you’ve forgotten System 6. Some very parsimonious assumptions were made about how much RAM would be needed for the accoutrements of the OS itself. But if you filled all available slots for Disk Accessories (15) and crammed your system with fonts, you could eat up the entire system heap and it would be very unstable, even unbootable. (I don’t remember if INITs and CDEVs also ate into the sys heap or not; if they did, there had to have been sysheap hell to pay for lots of folks).
e) Pretty sure I remember how to do it: Resetting the IBM 3090 session. A soft reboot of your own session. “IPL CMS” from the command line.
f) Pretty sure I remember how to do it: Force-quitting a hung app from the MacsBug debugger prompt. There was this little plastic thingie you’d physically snap into your computer on the oldies, whereas on the newer computers Command-Powerkey would bring up the prompt, no plastic widgets necessary. You’d get a “>”. And you’d type SMFA700A9F4, hit return, type PCFA700, hit return again, then type “G” and hit return. Hung app would be force-quit and you could save your work in other apps and restart the OS gracefully. Well, decently often for a non-protected-memory environment.
g) I have only dim memories: Setting the interleave ratios on hard disks. As unlikely as it sounds, hard disks could spin faster than the data-path architecture could keep up with. Early Mac hard drives had to be interleaved for best performance, meaning that what the computer perceived to be consecutive sectors were actually first, third and fifth, with the intervening sectors ignored untll the second spin. In fact I think the really old ones like the Mac Plus used a 3:1 interleave so it only tried to read from or write to every THIRD sector. Anyhow, it was something you did when you formatted your new HD.
h) I have only dim memories: Doing something to the hardware so it would recognize the extra RAM you just installed. Something about a resistor that had to be cut on the old Mac SE if you had the audacity to install more than 1 MB of RAM.
i) Pretty sure I remember how to do it: Resetting the PRAM before System 7. Instead of the ⌘-Option-P-R thingie, you’d hold down a key combo while selecting Control Panels. I’m pretty sure it was Command and Option.
j) I have only dim memories: Starting from a RAM disk. Something you could do if you had no hard disk and only one floppy drive but a decent amount of RAM (very common on the Mac Plus). There was a trick you could use that would make it load the entire floppy that the OS was on into RAM, then spit it out and you could use the floppy drive to insert diskettes containing your programs.
Yours? (PC users, not to mention Amiga and other platform users, fully welcome)