Focus on the stripy dot in the centre of this gif.
Don't worry - it's not just you. The yellow dot disappears, doesn't it?
The reason for this is "motion-induced blindness", a phenomenon discussed in a 2008 paper by Yale scientists JJ New and BJ Scholl.
The paper reads:
In motion-induced blindness (MIB), salient objects in full view can repeatedly fluctuate into and out of conscious awareness when superimposed onto certain global moving patterns.
When a small object is invariant despite changes that are occurring to a global region of the surrounding visual field, the visual system may discount that stimulus as akin to a scotoma, and may thus expunge it from awareness.
Essentially, the researchers argue that the illusion isn’t a failure of our eyesight, but an attempt by our visual system to remove peripheral moving objects to focus on the fixed and protect our sight.
Which is all very well, but it is freaking us out.