In my third area we were teaching this one sister and things were going pretty well...until we found out she was meeting with missionaries from the Jehovah Witnesses too. We found this out because one day when we showed up for our scheduled appointment with her, the JW's arrived 2 seconds after us for an unscheduled appointment with her. Talk about awkward.
The lady answered the door but didn't let us in. She didn't let them in either so it was all good. My companion at the time liked to bible bash (moron) so he started getting into with the JW's. I had a moment of weakness though and responded to one of their "attacks." They had said that Heavenly Father, Jesus Christ, and the Holy Ghost were all one person. They were very snarky and condescending about it and I couldn't help myself.
I got very smart with them and did not use my nice missionary voice. I read the account of Jesus's baptism from the New Testament and pointed out that we had: 1. Jesus in the water, 2. the voice of God coming from heaven, and 3. the Holy Ghost in the form of a dove. I held a finger with each person as they were mentioned and then looked at my hand in "shock" afterwards ("Oh my, would you look at that, three separate beings!"). I then also counted each of my fingers for them in a very...well, what's a good term for I treated them like they were a bunch of naive little children.
Looking back on that day I'm not super proud of my actions and know that is not how the Savior would have acted. The lady we were teaching did let us in that day and not the Jehovah Witnesses so that's one thing, but she didn't end up getting baptized at that time either. I hope my actions did not impede someone from being able to take advantage of the rich blessings of the gospel.
Moral of the story for current or future missionaries: the Lord wants us to speak boldly of his gospel and preach repentance with love, not bible bash and cause contention with bad attitudes.