Do you have LV versions 6.1 and 8.6 on the same PC? Are you trying to open the same VI simultaneously in both versions? Or did you open up an old VI in 8.6 then happen to save the file?
If you open up and old VI, it will recompile into the newer version. If you are trying to simultaneously open it in LV 6.1, you would probably have either file sharing conflicts, or conflicts of trying to have a VI of the same name in memory at the same time.
If you happened to save the file after you opened it up in 8.6, you won't be able to open it in LV 6.1 because the file version is too new. You will either have to find a backup version of that VI somewhere (hopefully you did make copies of your VI's for safekeeping?) or you will have resave it back to 6.1 which would be a multiple step process 8.6 to 8.0 to 7.1 to 7.0 to 6.1.
