Boomslangs are opisthoglyphous: the enlarged grooved teeth that conduct venom are near the rear of the upper jaw, under the eye, rather than at the front as in vipers and cobras. Because of that geometry the species was widely dismissed as not dangerous, and rear-fanged colubrids in general were treated as a category of mild snakes.
That view collapsed in 1957 when the American herpetologist Karl Patterson Schmidt was bitten by a juvenile boomslang at the Chicago Natural History Museum, kept meticulous notes on his own symptoms, and died the following day. His record documented progressive bleeding rather than the rapid neurological collapse expected of an elapid.
The venom is a haemotoxin that consumes clotting factors, and its effects are typically delayed by hours, which historically led to bites being underestimated until bleeding was advanced. The boomslang's mouth opens very wide, which is how a rear-fanged snake can engage a human limb at all, though how often bites achieve effective envenomation is not well quantified.