I can't remember the whole trailer. Basically a woman gets home just like other days, everything is normal, in other life, she somehow get home early and sees her boyfriend/husband cheating.
Do you mean Sliding Doors? Gwyneth Paltrow has two alternate realities which are driven by whether she managed to get on a tube train in time or not. In one, she finds her boyfriend at home on his own (his lover having just left) in the other, she catches them in bed.