Late nights at work are often an excuse used by a person who is playing around on their significant other, so checking up on a partner working extra hours with no visible benefit could lead to the discovery they are cheating. There are many others ways people cover the time they take with an illicit partner, but it matters little when their spouse recognizes the signs. The discovery of their unfaithfulness is the only thing that matters when it occurs.
For those who have been found out, the consequences are not always what they believe. Some of them might imagine their partner in tears, but others will envision walking away from the relationship. It is rare for a person to immediately forgive a partner, and some of them will opt for revenge. What happens next depends upon the two people in the relationship, and even they can be surprised by their own reactions.
Emotional upheaval is often the result of finding out a partner has been with another person, but some people are capable of pushing their feelings down immediately. For those who confront their partner, looking calm on the outside always helps their side of the situation. It might be best to wait a while before revealing the knowledge of cheating to a partner, and it will give the betrayed person the upper hand because they have had time to think about their own responses.
People caught cheating are often more prepared than their victims, so being able to face the situation calmly is what can turn the tables. Few people are able to do more than act out the pain they feel when they first find out, so taking the time to deal with the emotional havoc first will give them a better handle on facing the situation in a way that will eventually make their life better.