Grey's Anatomy/WMG

McCoy is an older version of McDreamy

McCoy is a doctor with Mc in his name, who is dreamy. So, logically, he would have to be Doctor McDreamy

Dr. Leonard McCoy is a descendant of McDreamy.

Same evidence as above, and this will spare Bones from having to live through the Dark Ages that happen between the end of Grey's Anatomy (the series will end no later than when television does) and Star Trek First Contact.

Reed and Percy died in the season six finale because Katherine Heigl quit.

Exactly What It Says on the Tin. IIRC, Percy's setup in the Mercy Westers' first episode involved confessing his secret love for Reed to Izzie. This track probably would have continued, with Percy mainly interacting with Izzie over the course of the season. Reed (who was not at all likeable) would have eventually become a Defrosting Ice Queen. But then Katherine Heigl went crazy and refused to show up for work anymore, leaving the two characters twisting in the wind. With their purpose lost, Reed and Percy were chosen to be killed to show that the finale was an episode where Anyone Who Has Done Jack This Season Can Die.

The hospital is built on top of a Hellmouth

Seriously, with all the bad things that KEEP HAPPENING there, it's the only possible explanation.