Does 石 have two dissimilar pronunciations? From CEDict:

MBP-WWG:~ Admin$ egrep "\t石\t" /Users/Admin/Downloads/cedict_1_0_ts_utf-8_mdbg.txt 
石   石   Shi2    surname Shi
石   石   dan4    dry measure for grain equal to ten dou 斗&lt;br/&gt;ten pecks&lt;br/&gt;one hundred liters
石   石   shi2    rock&lt;br/&gt;stone&lt;br/&gt;stone inscription&lt;br/&gt;one of the eight ancient musical instruments 八音[ba1 yin1]


Is the second one an error?

Is this what is meant by "variant readings" in  How many Chinese characters have multiple readings/pronunciations in Mandarin? ?
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.