I'm using Auto cad 2008 and noticed a strange behavior when I mirror my blocks. I normally create blocks using closed polylines like I've done countless times in the past except for now when I mirror my block and explode it my polylines turn into lines but if I simply copy the block without mirroring it and explode it, it's still a polyline. This does not happen with blocks that I’ve created in earlier versions of Auto cad without Microvellum running in the background. Does Microvellum have an effect on how blocks behave or perhaps it’s loading some kind of lisp in the background causing this problem?

Thanks 

Not sure about the answer to your question.  I did duplicate it the problem.  Here is a button macro that I use in order to quickly create pollylines.

^C^C_PE;M;W;\\;J;0;;

Great! Thanks for the macro.

But I'm still searching why this happens, I'll be testing it on a computer where we didn't install Mirovellum to see if the blocks behave differently.