This happened to me last night. When I went to bed I was too tired to feel like repeating mantras; I just wanted to sleep. As I was lying in bed I thought to myself "I haven't put any effort in becoming lucid during this day, and I am just too tired to do so now. There's no way I will have a lucid dream tonight.".

Next thing I know, I find myself inside a dream, fully aware that I am dreaming.

However, as you mentioned when you started this thread, convincing yourself that you will not become lucid with the intention of becoming lucid is what makes this technique difficult to use.