You can make smooth sandstone into smooth sandstone

Not really a bug, although sounds weird/pointless. I mean, is it really incorrect that 4 something turns into 4 same something? IMHO, that should be actually a valid case for almost any block type. It is pointless, yes, like pouring water from one bottle to another and calling it chemistry... but nothing breaks.

As Mortvert already mentioned, the recipe accepts any of the three sandstone types as source material, even any combination of them in the same process. Having 4 smooth sandstone just happens to be one acceptable combination. In the source code it is a single recipe which simply leaves the exact input sandstone type undefined. Note that this has no harmful effects, other than occasional reports in here. (Perhaps the 'fix' would be to add a note about this peculiarity in the wiki? EDIT: there already is: "Smooth Sandstone acts as regular sandstone; you can put 4 smooth sandstone in a square and make 4 of itself in a crafting table.")

The simplicity is also useful. Suppose you have 3 regular sandstone blocks and one smooth one. As it is currently, you can convert all those into 4 smooth sandstones. If this is changed, you're stuck with those 3+1 until getting one more regular sandstone block from somewhere. (Or, you can even convert 1+1+2 of all three types into 4 of smooth).

It is certainly possible to add some code to detect this particular case of 4 smooth sandstone, but is it really worth the effort? (I couldn't find any existing examples of a recipe with empty/null result defined in the code, or code that would be prepared to handle the case, so I'd say it needs a bit more coding than just adding one dummy recipe. Feel free to correct me if that is not the case.)

In that case just close it I didn't mean for it to be open this long anyway I just kind of forgot about it until I read my emails.

Smooth Sandstone has the same ID than normal Sandstone. Same with wool.

can confirm but i think that's actually kind of cool