Special relativity has several consequences that struck many people as counterintuitive, among which are:
The time lapse between two events is not invariant from one observer to another, but is dependent on the relative speeds of the observers' reference frames. (See Lorentz transformation equations.)
Two events that occur simultaneously in different places in one frame of reference may occur at different times in another frame of reference (lack of absolute simultaneity).
The dimensions (e.g. length, see Length contraction) of an object as measured by one observer may differ from the results of measurements of the same object made by another observer. (See Lorentz transformation equations.)
The twin paradox concerns a twin who flies off in a spaceship traveling near the speed of light. When he returns he discovers that his twin has aged much more rapidly than he has (or he aged more slowly).
The ladder paradox involves a long ladder traveling near the speed of light and being contained within a smaller garage.
Velocities do not combine by simple addition, but instead by a relativistic velocity addition formula.
Fast moving objects will appear to be distorted by Terrell rotation.
The inability for matter or information to travel faster-than-light.