Which safety issue prompted the major 2013 recall of Thames & Kosmos' CHEM C1000 chemistry sets?

In the world of scientific educational toys, product recalls can dramatically impact both brand reputation and consumer safety. In 2013, one major chemistry set was recalled across North America due to a specific safety concern that sparked debate about the balance between educational value and risk in science products. Test your knowledge about this significant product recall that changed how chemistry sets are designed and marketed to young scientists.

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Glass test tubes that could shatter under pressure
Calcium hydroxide bottles with defective caps that could leak caustic material
Bunsen burners with faulty gas regulation mechanisms
Thermometers containing mercury instead of the advertised alcohol-based solution