In this edit https://en.wikipedia.org/w/index.php?title=Barium_chromate&diff=1057528494&oldid=1037585482
User:MinusBot bot applied some AWB general fixes. These had the side effect of changing code inside a <chem> tag, in particular it changed
<chem>\mathsf{{Ba(OH)2} + K2CrO4 -> BaCrO4(v) + 2KOH}</chem>
into
<chem>\mathsf{{Ba(OH)2}} + K2CrO4 -> BaCrO4(v) + 2KOH}</chem>
turning a } into a }}. This broke the chem formula.
Chem blocks <chem> should be treated the same as <math> and no changes made inside the block. <ce> is a synonym for <chem> and should also be skipped.