When choosing a commercial Web site hosting company, beware plans claiming to be "unlimited." This claim is made in conjunction with the amount of disk space and the amount of data transfer (bandwidth) associated with the hosting plan.
The marketing may tout the sky as the limit, but the actual user agreement for the plan more often than not brings one right back to earth. These terms and conditions are carefully written in that they do not mention specific numbers but instead talk about "reasonable" or "normal," the definitions of which are entirely up to the company, not you or me.
Some companies have other limits that are hidden. HostGator, for example, limits the number of simultaneous processes to 25. The sites I build for clients render a page in under 100th of a second, which means the theoretical limit is 2,500 Web site accesses per second or 150,000 per minute. Those are huge numbers, unlikely to be reached by the typical small business. But it is a hard limit. (Note: I have pushed HostGator and found that at roughly 12-15 processes, a site can become unresponsive. That's still a big number in terms of site accesses.)
While I find the marketing practices underlying the "unlimited" pitch disgraceful, I do understand the problem. If a hosting company leaves the door wide open on a $5/month plan, in a jiffy they will be hosting the next version of YouTube.
It is always better to know what the limits are and to have the freedom to operate freely within those limits than to be at the mercy of a company that decides to cut you off because you stepped over some invisible line.
Read the fine print. Understand your limits.
Updated March, 2013.