I had one. It was a good thing for my marriage.
Actually, it happened before I got married. It was a friend of ours who we saw regularly.
He made no secret of his feelings for me and I, assuming that technically I wasn't doing anything physically wront, allowed it to carry on.
With hindsight, I didn't treat him very well at all.
To cut a long story short, things reached a point where they were getting very emotionally complicated, there was a drunken kiss. It was all getting out of hand.
I fessed up to DH (well DF) at the time. He was furious, obviously.
However, looking back. It was a good thing. I realised that I wasn't the perfect, never do wrong partner that I thought I was. Realised that it was DH that I wanted to spend my life with and that he deserved more respect than I was giving him.
He realised that maybe he did take me for granted and should probably treat me a little better too.
We are actually all very good friends again now (although there was a long time when we weren't).
But when all said and done, it was not pain free, lots of people got hurt and it was certainly not my finest hour.