Well, the reason I'm thinking resource limit is that this only occurs when busy, and then a couple of seconds later it works fine again.

The docs for select indicate that this is the select(2) system call, but the docs for that say the following:

...On error, -1 is returned, and errno is set appropriately;...

and lists the following errors:

• EBADF
  An invalid file descriptor was given in one of the sets. (Perhaps a file descriptor that was already closed, or one on which an error has occurred.)
• EINTR
  A signal was caught; see signal(7).
• EINVAL
  nfds is negative or the value contained within timeout is invalid.
• ENOMEM
  unable to allocate memory for internal tables.

... none of which correspond to the no child processes, and leaving me at a bit of a loss