The author discovered a bug in Linux 6.4 that causes it to hang on boot, occurring about once in every 1000 boots, especially if Intel hardware is used. Surprisingly, no one else has noticed this yet, but the author found it when their nbdkit tests, which use libguestfs, started hanging randomly. To fix this, the author ran guestfish in a loop to essentially bisect the issue. After many days of testing and writing a test harness that uses up to 8 threads, the author traced the problem to a regression in the printk time feature and successfully proved it by booting Linux 292,612 times before and after the faulty commit.
https://rwmj.wordpress.com/2023/06/14/i-booted-linux-292612-times/