There are some mistakes which many of us do and experience teaches us. If you are having a full time job, you just depend on that income source. In case of online business the approach should be different. You should never depend on one income source as there are chances that you have some problem with your single source of income. For example you are using Google Adsense for selling you advertisement slots on your blog and one fine day, you get an email from Google saying that your account is banned for some reason. You will be in trouble and you take some time to get other means to generate revenue. This becomes very critical if you are fully into online business without any full time job.
If you are a follower of Darren’s blog (Problogger.net) then you should be aware that Darren quit his full time job and now he makes huge money from his online business. He started with a single blog as a hobby, started making money with Google Adsense, the quit his job and fully spend his time on developing his blog. He then started selling ad space directly and stopped using Google Adsene on Problogger (he still uses on some of his other blogs).
If you have read his post on his different source of income you will understand that he makes money by selling e-books, posting jobs, selling ad spaces, affiliate marketing, etc. If there is a fall in one source of income, he can still survive with other sources and he gets time to put things back to normal. This factor is applicable to every person who is working full time online and making his / her living.
In my case, I do have a full time job now and have about 5 different source of income from my online business. I recently had a problem with my hosting of this site and my hosting account was suspended without any notice and I had my blog down for about 3 days and I did not have any income from this blog, but I had some other blogs on different server running and giving me some income. What I mean to say is even if its income source of anything else never put all the eggs into one basket.