1. Proprietary software could have a million bugs like this. You just wouldn't know it. They do not become less dangerous because they are proprietary, nor do security flaws become more dangerous because they are in open-source code.
2. Open-source software at least has the possibility of being looked at over and over. Proprietary code may be reviewed or not depending on the resources, interest, and monetization capability of that code. A possible review by all relevant coders in the world is always more review than by a limited team of programmers and analysts at one company.
3. The real problem with Heartbleed is the time that passed between code being written and a bug being discovered. That delay exacerbates the security problem. However, there will be some sort of statistical (probably Poissonian or normal) distribution of the time required to catch a bug since introduction into code. As with anything, there are outliers. Heartbleed with its serious and longstanding flaw must be considered an outlier unless shown otherwise. I have not seen evidence that this happens on a regular basis with any software, FOSS or otherwise.
I would appreciate it if future Slashdot discussions were let out through the upper orifice with some maturation period in the brain, rather than through the lower orifice after festering in the colon.