Geneticists! Assemble!

Righto, so I need help on PCR protocol.

The other day I spent about €2000 of lab materials doing a large PCR plate, only to have it's head asplode in the thermocycler.

The PCR reagents were Qiagen, the recipe was for normal PCR, the plate was a 96-well, sealed with one of those clear plastic strips as instructed by supervisor.

I'm assuming that steam pressure built up and made a mess, but any idea what I Should have done?

Kind of related. I'm in a debate with a stormfag, and he asked me to explain what I mean when I say "there is more variation within than between racial groups therefore race is a social construct." Truth be told, I have no idea what it means. What does it mean?