- PG only added checksums in 9.3

- his disks or RAID controller could be shit or he could be using some 'very cool' filesystem

- Some people turn off PG settings for performance that really shouldn't be turned off

- Some people think ECC ram isn't worth the money

- or overclock their CPU on anything other than a gaming machine

Usually it's shit hardware

https://wiki.postgresql.org/wiki/Corruption