Why are some examples of a chemical reaction not actually foolproof evidence that of a chemical reaction?13, 8th grade, and I'm doing my science lab report: 

The lab was a chemistry lab. We got a bunch of toxic chemicals and put pairs of them together, recorded the reactions, and wrote balanced equations for each of them and identified what kind of reaction they were. The whole name of the lab is "Evidence of Chemical Reactions", and one of the first analysis questions is "write a paragraph about the kinds of changes you would expect to see if a chemical reaction occurs." I answered this with things like bubble formation, change of color, change of state of matter, etc. Then the next question said "any one of these signs is not always a fool proof indication that a chemical reaction is occurring. give two examples to support this statement." 
what are two examples to support the statement that the commonly known examples of chemical reactions occurring are not always foolproof evidence that chemical reactions are occurring?