Cp: cannot create regular file: Invalid argument

• generally, unix prefers lowercase, definitely NOT all caps, READMEs are a special case and are uppercase (as with other special files like COPYING, LICENSE, etc..) - but otherwise, general (or "regular" as you call them) text files should not use uppercase.
• either hyphens or underscores, avoid spaces - even escaped
• somewhere in the middle: short enough that you can read it quickly and not waste space on your screen, but not so long that it will wrap around in a gui..

This turned out to be caused by a race condition. cp checks if the destination file already exists, and if not - overwrites it. The problem was happening because this cp command was being run twice in parallel, which caused the file in question to sometimes appear after checking whether it exists, but before the attempt to create the file. The strace output looks like this:

# Command was "cp a b"
stat("b", 0x7fff89510620)               = -1 ENOENT (No such file or directory)
stat("a", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
stat("b", 0x7fff895103a0)               = -1 ENOENT (No such file or directory)
# File b will be created at this point in time
open("a", O_RDONLY)                     = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
open("b", O_WRONLY|O_CREAT|O_EXCL, 0644) = -1 EEXIST (File exists)

Here's some bash code used to catch this:

#!/bin/bash

touch a

f() {
  while true; do
    rm -f b
    strace -o /tmp/cp${BASHPID}.trace cp a b || break
  done
}

cleanup() {
  kill -9 %1 %2
}

f &
f &

trap cleanup exit

wait

This same error can occur with mkdir -p or any other action that attempts to overwrite a file. Using flock can help avoid race conditions in cases like this.