UNIX targets reuse closed file descriptors which can lead to race conditions when a thread closes a fd while another thread is trying to read or write to the fd.

For example:

• Thread 1: file = File.open("1.txt") => ok (fd=10)
• Thread 1: file.read => checks file.closed? => false => ok
• Thread 1: is preempted by the OS
• Thread 2: file.close => LibC.close(10) => ok
• Thread X: File.open("2.txt") => OS reuses fd => ok (fd=10) 💣
• Thread 1: is woken by the OS
• Thread 1: LibC.read(10) => reads from 2.txt 💥

It might not be easy to trigger, and this should be a very rare scenario since a File or Socket should most often not be shared across fibers. But as always with threads: if it can happen it will happen, and when it does it will be pretty bad (data corruption, secure data leak, ...).