In zsh, what’s the difference between &! and &|

man zshall has the answer:

   source file [ arg ... ]
          Same as `.', except that the current directory is always searched and is always searched first, before directories in $path.

Example:

desktop➜  ~/pst  source /bin/env
/bin/env:1: no such file or directory: ^@^A^@^@^@X^Z@^@^@^@^@^@@^@^@^@^@^@^@^@\M-8q^@^@^@^@^@^@^@^@^@^@@^@8^@
/bin/env:2: parse error near `)'
desktop➜  ~/pst  . env
/usr/bin/env:1: no such file or directory: ^@^A^@^@^@X^Z@^@^@^@^@^@@^@^@^@^@^@^@^@\M-8q^@^@^@^@^@^@^@^@^@^@@^@8^@
/usr/bin/env:2: parse error near `)'

Sleep can fail if it is terminated during execution:

$ sleep 2
$ echo "$?"
0
$ sleep 2
^C
$ echo "$?"
130

Since sleep is an external executable, it is also conceivable that the fork or exec calls could fail, which would also cause bash to generate an error code >0.