Crystals are repeating arrangements of atoms bonded in a very particular, repeating way. Modern techniques include looking at the crystal with x-rays, so any serious discrepancies in the patterns would be noticed.
The contamination, they concluded, was not structural (e.g. as a silicon semiconductor might have random atoms replaced with phosphorus or boron) but rather even more minute amounts of acid could act as a catalyst - i.e. an extra component of a reaction that participates, but doesn't actually get consumed - hence can make a difference in tiny amounts. A decent way to think of catalysts is as chemistry's matchmakers - they help reactions happen; sometimes those would happen on their own but take more time, or sometimes they wouldn't happen at all (e.g. if there's not enough energy to react without the shortcut of the catalyst).
The contamination, they concluded, was not structural (e.g. as a silicon semiconductor might have random atoms replaced with phosphorus or boron) but rather even more minute amounts of acid could act as a catalyst - i.e. an extra component of a reaction that participates, but doesn't actually get consumed - hence can make a difference in tiny amounts. A decent way to think of catalysts is as chemistry's matchmakers - they help reactions happen; sometimes those would happen on their own but take more time, or sometimes they wouldn't happen at all (e.g. if there's not enough energy to react without the shortcut of the catalyst).
Hope that makes some sense.