Anthony Hoare writes:
The first American space probe to Venus [was] reportedly lost because FORTRAN cannot recognize a missing comma in a DO statement. In FORTRAN the statement
DO 17 I = 110
looks to the compiler like an assignment to a (probably undeclared) variable
DO17I = 110.
C. A. R. Hoare, "Hints On Programming Language Design," (Stanford Artificial Intelligence Laboratory, Memo AIM-224, STAN-CS-73-403, December, 1973), p. 14. http://www.eecs.berkeley.edu/~necula/cs263/handouts/hoarehints.pdf
This must have been Mariner 1, although most other accounts mention a missing hyphen or other symbol in mathematics, not code.