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TL;DR: You too can lose a weekend troubleshooting computer hardware, just like the old days.

Back in 2003, I got a Dell Pentium 4 computer that lasted me five years.  It was the last machine of two to three year upgrade cycles I had been doing since 1989.  Even with the five year gap, it would have lasted even longer had I not wanted a HP LP3065 30" monitor to match what I used at work; the video card in the Dell wasn't worth upgrading, especially since by then I preferred Apple machines.

So in March 2008, I replaced the Dell with my first new personal Mac, a high end Mac Pro.   The machine, a real beast, supports up to four 30" monitors, four internal hard drives, and two hardline Ethernet connections. They don't build 'em like that any more. Literally.    

(Don't drop it on your toes.  The case would survive having an anvil dropped on it.   I'm not sure how the anvil would fare.)

With a solid state boot drive added a couple of years ago, the Mac Pro has held up well, and even at six years old I am in no hurry to replace it.  So for its birthday, I had the flaky second video card serviced, and also ordered a couple of drive upgrades.  I got the machine back, installed Mac OS on a new bigger SSD drive, generally futzed with an almost-but-not-quite-perfect user migration, and I was back in business. 

Almost.

Checking the machine out, I realized I couldn't get the machine to accept the magic key to select the boot drive at startup.   Maybe it did accept the key -- but there was no screen to see the results.  I realized there are subtle evil technical reasons this could be a video card problem, which would require a trip back the Apple store.   

I prepared the machine to go back, including building yet another virgin boot drive (my third in a week) so I didn't take my data to the Apple store ... and the pre-boot screen started working.

After checking alternative keyboards, unplugging and plugging in monitors, swapping cables and reinstalling my real drives, I got it down to if my older HP 30" monitor was in the plugged into the lower video card and my newer Dell U3011 30" monitor was plugged into the upper one, I didn't see the Mac pre-boot screen -- the OS had to boot up before I saw anything. If I swapped them, the pre-boot screen did show on the old monitor even though it's dumber than the newer monitor.   Not optimal, but acceptable.

Mostly, just weird.