Throughout the whole movie, Tom Cruise
has different lengths of hair! Sometimes it's short or past his ears.
Users have written in saying things like "Why wouldn't his hair change? There are significant gaps in time throughout the movie. It takes place over a period of months and years, not days."
But, as Josh rightly points out, Tom Cruise's changing hair length is a continuity error, not a deliberate device. His hair changing over the course of the movie would not be a problem, but the length is markedly different even in the same scene at some points.