An earlier attempt at using the improbability drive, Starship Titanic, was also mentioned. In theory, the infinite improbability drive would make it infinitely improbable that anything would go wrong. It was not successful, however, ending in a "Spontaneous Massive Existence Failure." This was because, in these earlier times when the nature of improbability was less well understood, it was not appreciated that any event that is infinitely improbable will, by definition, occur almost immediately.
Unfortunately, human beings are accustomed to travelling at normality (probability 1:1), and can be fairly distressed by events around them whilst the improbability drive is working: losing limbs, turning into sofas, planets spontaneously becoming fruitcakes, nuclear missiles metamorphosing into sperm whales and bowls of petunias. The starship Heart of Gold was somewhat insulated against this by having an improbability-proof drive room, allowing the pilots to remain more or less normal during the flight.
The most important side effects of infinite improbability travel were that hyperspace express routes became largely obsolete - removing the reason for which Earth was demolished in the first Hitchhiker's book - and that the History department of the University of Maximegalon finally gave up trying to figure out the universe, as completely impossible things were increasingly commonplace.
Adams developed the notion of the improbability drive having greater causal (and narrative) effects in later books. For example: when Zaphod's grandfather discusses his great-great-great-great grandson's career-to-date he explains that he (Zaphod) cannot escape his destiny now the improbability field "controls you". This could be an early nod to the reverse-temporal abilities of the guide in the last book - although this may be good luck on Adams's part.