Does 石 have two dissimilar pronunciations?
Of course it's not an error.

Any respectable dictionary will have

石   shi2 

and

石   dan4

Variant readings refer to when one character has more than one pronunciation - based, usually, on different meanings.

Taking 石 for instance: when we're talking about rocks and stones - it is pronounced shi2; when we're talking about a specific measurement unit it is pronounced dan4.