Interference between 2 electrons
The mathematics of quantum mechanics predicts an unevenness of the observation, similar to wave equations in that there are points where high values of electron density/reaction and low (~zero) values occur. If you have places (~50% of the places) where "interference" has yielded zero electron density/reaction, then the other 50% of the places will have twice the expected (average) electron density/reaction.

The question should include the entire observation, the whole experiment. For example, it is easy to imagine a photon experiment where diffraction causes interference effects upon a photographic film: photons abandon their even spread by being diffracted, and don't go to certain places on the film, but do go (in twice the expected density) to other places. Two photons don't just cancel out in some places - they "cancel out" and also "add up" in other places. Oh, heck, just think of a sine wave (square its amplitude) plopped down on the film and where the high peaks are, the film turns dark and where the wave bottom is, nothing happens. Two photons, two sine waves; there has to be a slight difference in path so that when you superimpose them, they add up more in one place and subtract more in another.

That thought experiment was reasonably easy because we think of photons as sort of fuzzy, wavy, intangible things. We usually consider electrons to be touchable, therefore firm on some sort of basis, so how can they interfere and cancel out of existence? Well, the wave nature of electrons means that they have a spread in space which has some of the characteristics of a wave, and they can, under the right conditions, exhibit this effect strongly. Getting back to the photographic film experiment, electrons can be made to exhibit the same effects as diffraction does to photons. Where it becomes more difficult to understand is that while the photon (say, ~500 millimicrons wavelength) is much fuzzier than a silver ion (~0.25 millimicron diameter in AgCl), you would expect an electron to be, if anything, a bit smaller than an atom, since, after all, electrons (many of them!) will fit into an atom or ion. 

So you can see that a photon with a spread of 500 millimicrons will smash down over close to a million Ag+ ions, but can only activate one, but you might expect a little electron to be like a bullet and just hit one silver ion. How could you miss? Consider the bullet to be wobbly (wavy) enough to hit either one Ag+ or its neighbor. The faster it goes, the wobblier (wavy) it gets. And these waves interfere so that some paths just don't happen, and others are travelled twice as frequently - this doesn't mean that two electrons cancel out of existence or form a bi-electron.

At least that's the way I look at it without the comfort and brevity of mathematical equations. But  words only poorly describe the situation and it takes forever to try to explain.