What do you do when you catch someone lying?

Not a big lie, like a husband cheating on his wife. 
A small lie. Like a coworker or friend telling a story or answering your questions and they lie about a detail or deny something that you know for a fact happened. (I.e., "why did you post that on social media?" And they reply "it's not on there. Oh wait, I mean, it is, but it's been drafted for a while and I forgot it was there it posted itself." An obvious lie.) 
Do you call them out on it? Or just ignore it, they have their reasons for not being totally truthful? Do you trust them any less?