oh jeez the SD card story just gave me flashbacks.
I changed networks (uni to home) and the RPi running pihole would constantly keep my uni DNS settings after a restart.
Took me a good few hours to figure it out given that I thought the DNS settings were getting copied in from elsewhere on startup. I think i finally figured it out after i re-installed rapsbian from scratch and to my horror was booted in to the old system.
I had an SD card once that would not fail to write (i.e. kernel does not show write errors) but when you read back it gets the old data. Tried it on multiple operating systems with the same behavior.
It was a more expensive SD card because I see more issues with cheap ones.
But it seems to me that SD cards suck at all price brackets. Maybe even some of the expensive ones are counterfeit?
In my experience, Sandisk have a much better warranty policy than the other cards that have failed on me. RMAs get approved, UPS prepaid sticker for return, and replacement sent by UPS (in Europe, at least). The failure rate of their cards doesn't seem different than others I've used.
Yes in my case the pi would crash, and the SD card changes would revert. I don't know if its changed or not, but at the time you couldn't check uptime because the lack of a hardware clock meant it didn't know the time when it booted.
It took a while to figure out it was crashing everyday, probably due to the bad card, and the writes weren't taking. After more recent problems with other SD cards I'm trying the new SanDisk 8GB MSDHC Class 10 UHS-I U1 Industrial MLC micro cards after seeing them mentioned here.
I had a Neo Freerunner (2008ish time frame). So many SD card vendors ship (or shipped, then) bad/broken/out of spec SD cards whose controllers just barely work on Windows, Linux distros' drivers didn't work around the quirks, and probably the Freerunner SD port wasn't that great electrically. End result 8 out of 10 SD cards wouldn't work at all and one of the remaining two would fail silently later. I have still have a little box full of microSD cards.
I changed networks (uni to home) and the RPi running pihole would constantly keep my uni DNS settings after a restart.
Took me a good few hours to figure it out given that I thought the DNS settings were getting copied in from elsewhere on startup. I think i finally figured it out after i re-installed rapsbian from scratch and to my horror was booted in to the old system.
PSA: Bad SD card can make you feel crazy.