Thank you for the response Dave.
There are some reasons I do suspect otherwise. For instance, although it is rare, some of the markers that should be there are missing in my eye tracker data. In my specific case, I have 3 markers for each trial, indicating the onset of a gray picture, a photograph, and a white picture. For one of my participants, the eye tracker data stream goes from a white picture marker in trial 66 immediately to a white picture marker of trial 67, thus skipping the gray and photograph picture markers.
Additionally, if I calculate the time difference between the first gray marker and the first photograph marker for each trial (which should be 2000ms), I get the following results:
Min. 1st Qu. Median Mean 3rd Qu. Max. NA's
250 2000 2000 2036 2000 4467 4
So sometimes the difference between the onset of the first gray marker and the onset of the photograph marker is only 250ms, rather than the 2000ms it should be.