My problem when dictating to Siri is that I speak very fast, and not always clearly. I have always been aware of this, but with Siri, it's excruciatingly obvious.
True story: I was voice texting with a friend. We were both on our way to the same meeting. She was running late and needed to stop for something to eat. She asked me to explain that she was on her way. I was driving and dictating a text to her, and I wasn't taking the time to speak clearly. I told her that I was there would go on in.. What Siri texted was something like I'm here with a gun.
When she arrived, she ran into the room, very distraught. She asked if everyone was ok. I was surprised and said we're fine. She then burst out laughing and showed me the text she had gotten from me. While it was funny and was worth a good laugh, it was a painful reminder that I need to do a better job of speaking clearly when voice texting.
It's especially frustrating when autocorrect chimes in with Siri. That can really mess things up.