I've got my water changes off to a fine art now, i could do them blindfolded. I did make one big mistake one time though and it taught me a good lesson about getting complacent. I was filling up my tank with water from the mixer taps in the kitchen and unbeknown to me my son started having a hot shower upstairs. Most of the hot water got directed to the shower and left me with freezing cold water going into my tank. I didn't realise until my RTGG started freaking out. I soon picked up on what was wrong. Now if i'm doing a water change i ensure that no one turns any other hot taps on anywhere in the house. It's taught me to be on my toes at all times in this hobby, never get complacent.