Having worked in a pharmacy done this sort of bulk dispensing into bottles, (many years ago) I'd say this is a pretty poor excuse. The fact that probencid is normally dispensed a as tablet (not a capsule) does give a chance for this sort of cross contamination to actually occur.
When handing bulk tablets there is a small degree of attrition, and dusting. It is entirely conceivable* that this could end up on something else dispensed. If the contaminant was in a capsule, then the only way for contamination would be a broken capsule. (which does happen but rarely)
*Of course the best practice drilled into me would involve hand washing after any sort of bulk dispensing
Its just plausible enough with the paper trail in place to give reasonable doubt, though this statement is without looking at the levels found in the urine. I don't think I have seen that information.
** It has been probably ~28 years since I last worked in a pharmacy, things do change, but the essence of bulk dispensing stays the same.