Computers now play a dominant role in experimental control applications, but everyone knows that operating systems and video/audio hardware are unpredictable and unreliable to some degree.  You shouldn’t assume that stimuli appear when you want or expect them to, or that stimuli and triggers leave the experimental control computer at the same time.  Nor should you take the word of the software developer that there are no timing problems in their software.  Timing problems are often introduced by the operating system, the video or audio hardware or another device installed in the computer, meaning it is virtually impossible for the experimental control software developer to guarantee timing.  It is always best to test timing independently to be sure that events are occurring when they should.