What are some of the pitfalls of having an affair with an engaged woman?

I am doing the above ^_^ There were at times that it wasn’t so and then on and off. In her words we were dating, however, my definition must be different. Once I unknowingly dropped her off at fiancés work, I felt so bad ‘cause I was in that situation before, I withheld affection then it was deciced that we were not dating. So I think we have complete honesty. Between all the whirling feelings of jealousy, etc. would you at least list some early warning signs of get the Hell out of there