The first thing they teach you in business school is to assess risk, and to avoid it if you can't control it. Controlling it involves insurance policies of one sort or another. If your applications are truly mission-critical, you can't risk their failure. If they do fail, you can't risk the loss of business that will follow.
You can buy business-discontinuance insurance if you wish, but you should certainly look for another form of insurance: a dependable support vendor for your software. This applies both to open source and to proprietary software. There are large, obvious vendors of such support (Red Hat and Oracle, if you are reading the news), and there is always IBM. The most expensive support plans promise to answer the phone 24/7. Only you can decide if you need, and can afford, such high-end plans.
If you are using open source applications for mission-critical work, you have probably not simply downloaded them from the Internet and compiled and installed them. You probably found a vendor who would provide a finished, tested version of the open source software, and charged for this added value. And this vendor probably offers support plans, too.
You benefit from such vendors by paying for their work in assembling a reliable product. They benefit not only from your payment but from knowing that, when you ask for support, they know intimately what is inside the software they delivered.
You can find support paid plans that cover freely-downloadable software, and although it is possible to find free-lance (third-party) support vendors, it is highly likely that a paid support plan comes from the commercial side of the open source project that you downloaded from. Don't fail to check out third-party support vendors. One benefit of open source is that you are freed from vendor lock-in, and can buy your support where you believe they deliver the most value.
This was first published in November 2006