- Don't cover the wort when cooking!
It will over flow right after you add the hops and part of it will pour outside of the pot. As soon as the wort starts bubbling up keep that lid off up until the whirlpool.
- Always have spare yeast ready!
It could happen that the main fermentation doesn't start because the yeast died. If you have no spare yeast ready in time bacteria will enjoy the wort sugar and ruin the beer.