I am not sure that there are/were "high profile failures", but serious bugs for sure, not just in Mathmetica but also in the calculators you mention; ser eg here: https://mathoverflow.net/questions/11517/computer-algebra-er...

Now being open source does not prevent such errors. What I think is far more important here is open issue tracking: companies like Wolfram may not be exactly eager to let you know that there are serious bugs in their products (and what they are) unless they must. Being able to fix the bug yourself can of course be a real perk for some user's, but realistically, it's out if question for most.