As you probably already know, the process of cellular respiration has the following chemical equation: C₆H₁₂O₆ + 6O₂ --> 6CO₂ + 6H₂O + Energy.

The water that is formed in this equation largely comes from the final step of cellular respiration: oxidative phosphorylation. Oxidative phosphorylation is catalyzed by high energy electrons released by NADH and FADH₂. Part of this process is the electron transport chain, which uses coenzymatic activity to produce several protons. These protons react with oxygen within the mitochondrial matrix, forming water.

There are several reasons cellular respiration might produce less than 6H₂O, but a compelling one is that the levels of NADH or FADH₂ were not high enough for an efficient oxidative phosphorylative process.

This does happen in nature; for instance Alzheimer's patients can produce subpar levels of NADH. As you might expect, this can mess up mitochondrial function, and lead to reduced anti-oxidant protection. Alzheimer's patients sometimes take NADH-enriched vitamins to help compensate for this problem.