I don't think so, at least not that are claimed to have occurred by a large group of people. It may also be useful to note that most people who deny the Armenian genocide or the Holocaust don't say that nothing happened, but rather that the numbers were inflated, the goal was something other than ethnic cleansing, or something of that sort - not saying that something bad didn't happen, but trying to downplay its importance and remove the "genocide" label. Obviously, this isn't correct - the Armenian genocide was almost certainly a genocide (though this is contested by some), and the Holocaust was absolutely a genocide (though let's not overlook the other populations who were targeted in addition to Jews, including the Roma/"gypsies", the physically handicapped, and homosexuals). My point in saying this is that, if you do find an example of a genocide that "legitimately didn't happen," it's probably going to be debatable - likely an instance in which many people died, but in which numbers and/or ethnically-based motives are contested. You won't find an instance in which people strongly claim that a genocide occurred, but it was totally fabricated out of thin air.

I don't think this is what you're asking about but not all mass killings necessarily have to be genocides, depending on the definition that you use. For instance, the UN Genocide Convention does not include groups or victims defined by their political position or actions (i.e. mass killings of political groups are not genocides even though they share similarities in their causes, organizations and motives as mass killings of national, ethnical, racial or religious groups).

On p. 60 of Barbara Harff's "No Lessons Learned from the Holocaust?", there is a handy table that distinguishes politicides (i.e. mass killings of political groups) from genocides.

The only example I can think of where this might be the case would be something from antiquity where we have very few sources and it's not entirely known what's history from what's just myth or exaggeration. Mind you, that certainly doesn't mean nothing happened. Just that we don't necessarily know. The fact that these type of events are not usually referred to as "genocides" in the first place makes this even more dubious. However, quite a few ancient and classical texts are certainly known to use some very bombastic language when describing destruction and mass killings in ways that make events sound almost like what we might call "genocide".

There are mentions of what might possibly qualify as a "genocides" in the Old Testament. The Midianites (Numbers) and Canaanites (Joshua) come to mind. The Siege of Jericho, in particular, has some very genocidal sounding language surrounding it. Example:

"And they utterly destroyed all that was in the city, both man and woman, young and old, and ox, and sheep, and ass, with the edge of the sword." (Joshua 6:21, KJV).

I know a lot of archeology work has been done on Jericho, with an attempt to discern if such a siege took place or not, however, I'm not aware of any up-to-date or conclusive results of anyone's work.

I don't know if any of this really answers your question, but I suppose my point is, that there are recordings of mass killings (that might possibly qualify as genocide if taken at face value) that are in doubt to varying degrees.

A few points about genocide:

-The use and definition of the word "genocide" has always been strongly influenced by political motives.

-The legal definition of genocide is a result of political bargaining.

-The definition used in the UN charter is rather vague, so a lot of bloody situations could be categorized as genocide.

Stalin made sure that the mass killing of class (i.e. political) enemies was not included. On the 11th of December 1946 during the 55th plenary session of the UN, the (preliminary) legal definition still included the mass killing of political enemies. On the 9th of December 1948 the Convention on Genocide is adopted by the UN General Assembly (resolution 260 (III) A). After two years the mass killing of political enemies is not part of the definition anymore:

"Article 2

In the present Convention, genocide means any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such:

(a) Killing members of the group; (b) Causing serious bodily or mental harm to members of the group; (c) Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part; (d) Imposing measures intended to prevent births within the group; (e) Forcibly transferring children of the group to another group."

It means that Mao and Stalin were able to organize the killing of millions of their political adversaries without being punishable by international law. The millions Hitler killed were punishable. There is no rational argument for this difference in the legal status of the killing of a group of people, other than the political influence of the communist bloc on this legal definition.

The Convention on Genocide furthermore forces the signing parties to act and to punish the responsible persons and political entities. This is an interesting part of the convention: by claiming that a belligerent during a war is committing genocide it forces the Security Council to act. As a consequence that particular belligerent is instantly an international pariah and its leaders are unable to perform their duties.

The Kosovo Liberation Army or UCK claimed in 1999 that the Serbian Army was committing genocide on its people and provided forged evidence. They claimed 100.000's were killed. Later it turned out that 1000's were killed and that the acts of the Serbian Army and paramilitary forces could not be seen as genocide. They surely misbehaved and did bad things, but it was no genocide. Nonetheless NATO send its warplanes and ground troops to deal with Milosevic and the Serbian ambitions in the Balkans.

It was later established that UCK fighters and its leaders were engaged in criminal activity. http://en.wikipedia.org/wiki/Hashim_Tha%C3%A7i The investigation about the trade in organs from Serbian prisoners is still being conducted, but it doesn't provide any confidence in the trustworthiness of the claims of genocide. Kosovo got its independence a decade later anyway, after a this highly questionable string of events, starting with the claim of genocide.