Fair question, perhaps recursion? I could imagine an unscrupulous person creating the 'mega-startup-accellerator' and having as its first round a startup they fund which is required to acquire half the startups of each subsequent round. Thus creating the worlds most successful startup accelerator where nearly half the companies are guaranteed to have an "exit".

In this case it is more like having the kids grow up and being successful than anything nefarious. And YC gets to put 'acquired Apr '14' in the box of outcomes.