Anything that makes it "very easy to do wrong" is not a good idea in flight software. A missing semi-colon in FORTRAN once completely killed a space mission.
I remember reading semi-colon in the report ;-) . I can't find the PDF that I believe I read this in, unfortunately, and I'm not sure if it's publicly available. However, this wikipedia article points to a hypen and suggests that there's some folklore involved, and that revisions have been made in the past: http://en.wikipedia.org/wiki/Mariner_1 .
Edit: Andrew, thanks for pointing out my ignorance, I've never coded FORTRAN, and have been thinking it's a semicolon this entire time. What fitting irony. Sort of makes for an even better story now. >_<
this seems like it may be definitive (well, it's a quote that gives a reference; it also fits with what i remember from the news at the time, which is that it was a do statement and related to the parsing of spaces, but i have no idea how reliable that is): http://catless.ncl.ac.uk/Risks/5.66.html#subj2
if it's correct than it was actually a decimal point instead of a comma.
