If you have perfectly pure distilled deionized water, nothing will happen if you reboil it. However, ordinary water contains dissolved gasses and minerals.
The chemistry of the water changes when you boil it because this drives off the volatile compounds and dissolved gasses. There are many cases where this is desirable.
However, if you boil the water too long or reboil it, you risk concentrating certain undesirable chemicals that may be in your water.
POTENTIAL EXPLOSIONS
It is a huge risk if you reheat your tea in the microwave.
Microwaves heat water molecules very quickly. At times, the molecules get heated before they can turn into vapor. When you take the cup out of the microwave, it begins to cool, and the water vapor suddenly goes *poof!*
Stir the water before reheating to prevent this. Most people do not do this, or they don’t know they are supposed to do it.
TOXIC CONCENTRATION INCREASE
Tap water is not that clear as it looks. The Environmental Working Group found 316 chemicals, such as:
Arsenic, Chlorine byproducts, Fertilizer, Plasticizers, Radioactive isotopeFluoride
These chemicals cannot be eliminated by boiling, unlike microorganisms. When boiling water with those chemicals, you are making the concentration of the toxin worse.
The water starts to evaporate, while the toxins stay put, making the water even more dangerous. So, the more you boil it, the more dangerous it becomes.
High levels of these chemicals can be hazardous.