When string->number is given a number in scientific notation with a very high exponent, it errors with "value out of range."
I don't know if that is acceptable, but what seems unacceptable is that it errors even if the string contains further characters and is thus not a valid number; R5RS says that #f should be returned when the string does not contain a syntactically valid number, and R7RS adds explicitly that string->number never signals an error due to the contents of the given string. Apparently MIT/GNU Scheme has the same bug, as reported by 'ecraven' on #scheme. From other systems I tested, Racket, Gauche, Gambit, and Chibi return +inf.0 for numbers with a too high exponent; I'm told Chicken also does so when the "numbers egg" is loaded. Taylan