I have never had a manifestation backfire, HOWEVER, I have had mediation sessions get invaded by things I didn't want when I was sloppy with my affirmation statement. Oh and if I'm in a bad or negative mode I will not go near any grids I have setup What Shu-Man is saying is certainly good advice. I think practices like that help one walk through the process of being thorough in their imaging/creative process. Again I'll note here: One may 'Discreate' as well as "Create". I think that is a great defensive tool.