Have you gone to use your parallel pipes tool, but the button has gone missing from the ribbon?

There is a bug in Revit 2012 which causes both the parallel pipes and parallel conduit tools to disappear from time to time.

The quick and simple solution is to use the hot keys PE for parallel pipes or PO for parallel conduits.

Once you have run the tool from the hot key, the button will also re-appear on your toolbar.