Wordpress limit of 140 files - What causes the limit?

I made a simple Wordpress plugin which adds files to users but I hit errors when I try and add more than 139 files to a user.

Is there some sort of built in limits with php or Wordpress that could be causing this? I say this only because there is nothing in my code to put any limits in place.

I've done a bit of searching but Google doesn't bring up anything and I'm surprised I'm the only one who hit this problem.

via Chebli Mohamed