One thing I've noticed from babysitting is that every house runs differently. One issue is with knocking before entering the house. I remember once hearing a halacha that you are supposed to knock before you enter a house even if its your own house.
One person I babysit for follows this halacha and she always knocks and waits to hear my response before she comes in so that I should be ready for her. Most of the time its night time and the kids are sleeping and I sit by the table doing homework or whatever it may be and its quiet so I hear the car coming so I know she will knock and I start to go to the door so she'll hear my footsteps and know she can come in.
A different person I babysit for, I have the key to the house so I can let the kids in after I pick them up from the bus stop and I stay with them for a while after the mother comes home. For some reason I never hear the mother come in, its all of a sudden I see her there and I'm like whoa where did she come from. Then I start to think of what I just said within that minute, maybe I did something wrong I didn't want her to overhear. (Like letting the kids have candy).
Then there's a different person I babysit for and she has a baby, so she doesn't want to come to the door with the baby, she's my next door neighbor, so she calls me when she wants me to come over, she leaves the door open a bit and goes to the living room with the baby till I come. So I usually knock on the door or ring the bell and she says I can come in.
Today, I don't know what I was thinking, but I saw the door open so I figured she opened it for me so I should just go in, I thought maybe the baby was sleeping so I didn't want to wake her by ringing the bell. So I go in the house, and I'm like "Hello". I thought she heard me but apparently not. All of a sudden she sees me and she's like "wow you scared me!". I'm like "oops". Everything was alright, but I think from now on I'm going to try to stick to knocking before I come in!