Another puzzle

I have blogged about the broken state of chemical information and the lack of semantics in current commercial chemical software and drawing tools. This is exemplified by the amount of incorrect structures received by publishers (and I have talked to several).
What is the most common chemical substance received by a chemical publisher (or at least the Royal Society of Chemistry, thanks to Colin Batchelor)? It’s not water, or a common solvent. It’s an error.
If you KNOW – i.e. I or Colin or someone else have told you – please don’t answer. But otherwise please guess.