Every once in a while you see your eyelids close in pain when a bug flies into your eye...
... hmm ... actually there are two more :
1.) when lower blood-pressure is in brain for an instance (standing up too quick), you see some little whytey-rainbowy dots flying around for a brief moment.
2.) Sometimes, you notice coagulated proteins floating around in the intraocular fluid. (its perfectly normal and they normally dissolve after some time again).