First OS install ever:
Early spring 2009, asheville nc - buys new amd desktop at best buy. Plays in windows for around a week, remembers hearing about linux somewhere, reads about linux, installs ubuntu karmic. Almost everything breaks. Reads more about linux and spends 48 hrs day and night trying to fix. some drivers don’t exist yet, fails miserably. Returns to best buy with tail between legs and returns desktop. Service desk boots up machine, techs giggle, returns home with new windows desktop. Summertime - decides to move in VW van for awhile and tour country, drops of computer at a friend’s house in baltimore. Fast forward 30 states and ten months - returns to baltimore may 2010 to find friend’s mom has borrowed computer and infected it with virus(s) - unusable - decides to try linux again. Installs ubuntu lucid, everything works! Lives happily ever after.
Spending about 48 hours with only 4 hours of sleep, trying to install Gentoo ! That was just plain stupid and I won’t attempt doing it anytime soon.
It can be done! I’m running Gentoo on my workstation and a server. Once you recompile your entire system using distcc on all your computers in parallel, you’ll never go back!
With dd I have set of=/dev/wrongdrive and learned to triple-check what of points to.
I ran CyanogenMod nightlies on my Galaxy S2 and one day the nightly had a bug in the kernel causing the flash to be unreadable. Moral: Don’t run nightlies 
The most exotic one:
On my old ThinkPad without SSH enabled I accidentally wrote to the wrong i2c address wiping the EDID eeprom of the built-in screen. This soft-bricked the screen but it was recoverable with some patience and a lot of research in what EDID actually was. Moral of the story: your monitors EDID may be writable.
When Ubuntu 7.04 or 7.10 came out for laughs I attempted to install it on a old Pentium III, quickly found out how awful older ATI graphics cards were supported and had to force no-hardware acceleration… (this lead to my later preference of light weight Linux distros)
A few years later I tried to get a WinTV HDTV tuner to work on Linux, somehow the revision I bought had a different chip and you had to compile the driver yourself… by accident I overwrote the folder instead of pointing to the subfolder. Ended up finding out I had a older Pinnacle HDTV stick which was actually based on the WinTV 950 and fully worked on Linux 
(non-Linux friendly WinTV 850 remains plugged into a Windows box)
No, dd stands for ‘disk destroyer’ 
My worst foul up was installing the Nvidia proprietary driver - and watching everything turn blue.
http://wiredrevolution.com/ubuntu/fix-blue-tinted-video-in-ubuntu
That wasn’t the bad part, though. When I tried to uninstall the proprietary driver, I completely f’d up nouveau and was left with no graphics at all. Nuked and paved that mess…
Done that, DDed my Downloads/music/mods partition, but it is fine. I used photorec to recover everything to another drive, then i DDed that one as well.
It was a weekend of frustration trying to get any OS to work on my machine at all