Don't worry, that's not an alarming experience. You have a literal blind spot - a part of your retina with no photoreceptors - because this is where the optic nerve connects.
Your brain fills in missing information with the surrounding pattern (this is why you generally don't notice the blind spot). So small items that break up that pattern can seemingly disappear if you look in the right place.
Here's a demonstration of the blind spot, as well as some other "eye tricks":