Enemy Response Fail #1 - Pylon is Building - Send One Worker to Kill Probe
Some people try to solve the problem by sending only one worker to kill or follow your probe. One worker cannot stop anything. Probes are fast. If they send one worker to kill your probe, just queue it up to run a big circle in their base, and build the pylons and cannons as the other worker chases you.
Enemy Response Fail #2 - Pylon is Building - Send One Worker to Kill Pylon
If they send one worker to kill the pylon, laugh at it. It won’t do enough damage in time. They can’t stop you unless they commit more resources to the task.
While their probe is attacking the pylon, use your probe to attack their probe. If they don’t pay attention, you will kill it. If they do, they might try to fight you. In that case, either kill them (if you are ahead on damage) or run around in a circle.
Enemy Response Fail #3 - Pylon is Building - Send All Workers to Kill Pylon
Some people overreact. They send ALL workers. Workers collect minerals at a rate of about 0.8 minerals per second. So, if they have sent 10 workers, that’s 8 minerals per second. If you can waste their time for 3 seconds, you pretty much break even when you cancel the pylon.
If they send all the workers to kill the pylon, start building another pylon. Just watch the first pylon and cancel it before it dies. Usually, I will build this second pylon far away to make sure their probes have to travel and waste maximum time.
If they are killing your pylons, you may have to bail on the strategy eventually. But you can really damage their economy by forcing them to stop mining. More importantly, you damage their mind set and their rhythm. Also note that YOU won’t have to stop mining while this happens, so you can use this time to start building probes and catch up.
Enemy Response Fail #4 - Pylon is Building - Send All Workers to Kill Probe
If they send all of the probes after your probe, just laugh and run their probes around (or even out of) their base. As Borat said, "They will never get this!" There is no greater comedy than watching a comet tail form behind your probe as it trails angry workers around the enemy base.
Enemy Response Fail #5 - Pylon Completes - Send All Workers to Kill Pylon
This is my favorite mistake. When the pylon completes, immediately start building cannons. If they send a ton of probes to kill the pylon, good. The main goal is to get your cannons to complete.
Most people think that if they can take out the pylon, the cannons will power down. This is true. But, once the cannons are completed, it is really easy for me to build a bunch of pylons and get the cannons to power up again.
Anyone who has seen the Korean 4 warp gate knows that you can’t stop ALL the pylons from completing. One of them will finish. And when it does, the cannons will power up again. I have won many games like this.
The moral here is focus on what kills you... a pylon can’t shoot you, but a cannon can. It’s easy to build pylons: they are cheap and they complete quickly. Cannons are more expensive and take more time. Without the cannons there is no rush. So don’t let the cannons complete.