The MOST common threat that impacts a business's ability to function
normally is power. Power interruption cause more business interruption than any other type
of event.
The second most common threat is Water such as flood, water damage from broken pipe,
leaky roof, etc...
Threats will be discovered while doing your Threats and Risk Assessments (TRA).
There are three elements of risks: threats, assets, and mitigating factors (countermeasures,
safeguards, controls).
A threat is an event or situation that if it occured would affect your business and may even
prevent it from functioning normally or in some case functioning at all. Evaluation of threats
is done by looking at Likelihood and Impact of possible threat. Safeguards,
countermeasures, and controls would be used to bring the threat level down to an
acceptable level.
Other common events that can impact a company are:
Weather, cable cuts, fires, labor disputes, transportation mishaps, hardware failure,
chemical spills, sabotage.
References:
The Official ISC2 Guide to the CISSP CBK, Second Edition, Page 275-276

Section: Network and Telecommunications
Explanation/Reference:
Bootstrap Protocol (BootP) is an Internet Layer protocol that enables a workstation to boot without requiring a hard or floppy disk drive. Reverse Address Resolution Protocol (RARP) is a TCP/IP protocol that permits a physical address, such as an Ethernet address, to be translated into an IP address. Address Resolution Protocol (ARP) is a TCP/IP protocol that permits an IP address to be translated into a physical address.
Classless Inter-Domain Routing (CIDR) is a new IP addressing scheme.
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, 2001, John Wiley & Sons, Page 88.