Photo or pattern sensitivity, motion sickness and reactions to low frequency sound

Small proportion may be sensitive to flashing/flickering light. May cause seizures or other serious physical effects.
Some viewers may experience motion sickness due to hand-held camera shots.
Some might be affected by low frequency sounds. 
Responsibility of the film producers to warn viewers of such possible side effects. 
Usually, these effects aren't usually considered when rating work, but if it is likely to affect a large number of people, the BBFC may advise the distributor that the appropriate warnings are put in place. 
Extreme cases - assurance regarding the display of appropriate warnings may be required as a condition of classification.