[GOPJ]

there is no way of observing a sub-atomic particle without affecting the particle, directly. In other words, if you were to imagine the electron as a ball, in darkness, the very act of shining a hypothetical “light” to detect its reflection off of the “electron” has an effect on the electron itself. That is the scale we’re talking about here.

Which begs the question - what is the "light" we're shining?

[James C. Bennett]

A good question. Perhaps that is the source of the apparent interference, as well as the behaviour of the slits themselves being changed each time a particle passes through it, to affect the following particle.