Canon rush...

All this can be avoided by simply scouting.

Learn what to look out for in opponent's base. If their buildings or lack thereof seem strange, then do a cheese scout in and around your base!

Terran - bunker near canons
Protoss - canon near canons
Zerg - spine crawler near canons

Unless you spot the pylon early, then just send 4 or 5 workers to kill it off.

It's also a good idea to proxy your opponent, if they are/tried to proxy/rush you.