It's not a big deal in the sense that it's easily reversed, but it is a big deal in that it means the tool is unpredictably unhelpful. Of the properties that good tools in my workflow possess, "unpredictably unhelpful" does not make the top 100.

When a tool starts confidently inserting random wrong code into my 100% correct code, there's not much more I need to see to know it's not a tool for me. That's less like a tool and more like a vandal. That's not something I need in my toolbox, and I'm certainly not going to replace my other tools with it.