After a disappointing experience with Linux on my K6-2 build, I thought I'd give OpenBSD a shot. I heard it had solid support for older hardware; I had also tried it a few times in the past, both on real hardware and in VMs, and concluded it was fine.
Sure enough, I installed it, then got X11 and Lynx running without any major issues. From there, the OS made it onto three of my newer PCs. I tried to actually use it, and that's where the show-stopping issues cropped up.

• If I start QEMU with 512 MiB of RAM allocated to the virtual machine, it crashes with the text "qemu-system-x86_64: cannot set up guest memory 'pc.ram': Cannot allocate memory". Most forum threads on this issue provide the unhelpful suggestion to "install more RAM"; however, QEMU crashes regardless of how much RAM is on the host. Here are the first few lines of top on one of the affected systems:
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  1load averages:  1.70,  0.98,  0.43    gemcookie.my.domain 08:19:22
  249 processes: 48 idle, 1 on processor  up 0 days 00:03:19
  3CPU0:  7.2% user,  0.0% nice,  3.4% sys,  0.4% spin,  1.0% intr, 88.0% idle
  4CPU1:  7.4% user,  0.0% nice,  3.0% sys,  0.4% spin,  0.0% intr, 89.2% idle
  5Memory: Real: 401M/2263M act/tot Free: 13G Cache: 1090M Swap: 0K/4339M
  

  I clearly have more than enough RAM to start a virtual machine.
  Another suggestion involves writing a value to /proc/sys/vm/drop_caches. This isn't an option, since OpenBSD has no /proc directory!
  I can confirm this issue on two machines running OpenBSD/amd64 7.6 and one running OpenBSD/i386 7.3.
• I can't get any acceleration out of my Quadro FX 2700M, even though the nv driver should support it. I ran Xorg -configure in an effort to rectify this, only to get a nonsensical error message:
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  1c2d-openbsd$ Xorg -configure
  2Unrecognized option: -configure
  3use: X [:<display>] [option]
  

  ...
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  1-configure             probe for devices and write an xorg.conf
  

  ...
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  1Fatal server error:
  2(EE) Unrecognized option: -configure
  3(EE)
  4(EE)
  

• valgrind crashes hard on startup.
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  1memcheck-amd64-openbsd[83754]: pinsyscalls addr 3806a478 code 20, pinoff 0xffffffff (pin 0 0-0 0) (libcpin 0 0-0 0) error 78
  2Abort trap
  

  I can confirm this issue on two machines running OpenBSD/amd64 7.6.

Any ideas as to what could be wrong? I might be running out of non-Windows operating systems to play with. Linux isn't looking much better, with the nouveau driver causing segmentation faults on pre-unified shader cards.