If you make a 1 unit high slope and place a balancer half over the edge, it will spawn a small amount of rock under the balancer. This can be mined and the process repeated to glitch in free rock.
Make a 1 unit high ledge with a slope.
Place a balancer half off the edge at the top.
Rock will spawn under the balancer.
v0.7.6a (b466)
Windows
InfiniteRockBug.save
that's an awfully tedious way to get a few bits of rock... lol
btw, ghost lifts will do a similar thing for deleting rock on a slope (you don't even need to build it)
rails also do this, its very intentional for a bunch of things and is particularly useful for terrain thats basically "99% flat"
given this also goes in-hand with the small chance of excavators voiding materials as well, usually in very tiny amounts, its really not a huge deal in either direction, just one of those things that isnt really avoidable until computers can handle counting individual molecules