Why do I get hot water in my shower when someone flushes the toilet?
This is caused by a pressure drop in the cold water line. Toilets use only cold water so when you flush the toilet, it calls for cold water and thus causes a drop in cold water pressure to the shower. You can fix this by installing a pressure balanced shower faucet which limits the change to +/- 2 degrees. These types of fixtures are code in all new homes.