I bought a jar of honey a few months ago, and left it at the back of the cupboard. When it resurfaced a few days ago, I saw that the jar contained a lot of bubbles, to the point where the lid was bulging. I assumed that it had somehow fermented, particularly since it came from a local grower, where taste can be expected to take precedence over sterilization; this answer seems to support my theory.
On the other hand: the jar was still (to inspection) airtight, and when I opened the jar there was no gas release; the honey gently oozed over the top, and remained inert. The bubbles have remained in the same place over the last week, and the honey tastes exactly as normal.
Clearly some sort of chemical reaction has taken place; is it classed as fermentation, and is it harmful?
(It is almost impossible to take a picture of this; the bubbles are equally distributed throughout, giving the honey much the apparance of the interior of an Aero bar. If you were eating it with your eyes closed, I don't believe anybody would notice any difference).