Continuity mistake: Caleb hits Melanie with a piece of lumber and she falls to the ground. She falls with her feet towards Caleb, but in the close up (and each scene after) her head is now towards Caleb.
Continuity mistake: After Caleb thinks he has killed Melanie, he wipes his right hand across his T-shirt several times and leaves one bloody handprint. When he gets home, there are now several bloody prints from his right hand on the T-shirt.
Continuity mistake: In the 1994 flashback where Bree introduces herself to Mary Alice and forces her son Andrew to apologise when he stole a ornamental garden frog, the position of the frog changes between shots as Bree hands it to Mary Alice.
Chosen answer: Poker. They have been playing that this whole time, once a week with everyone who was invited that lived on the Lane.