Step 1. Devise an awesome failover scenario with boot-from-SAN, SAN mirroring, automatic DNS selectors and all sorts of fun stuff.
Step 2. Add lots of hosts entries to your servers to cover up problems with your DNS usage.
Step 3. Forget about Step 2.
Step 4. Have a failure that exercises the engineered solution in Step 1.
Step 5. Spend the next several hours wondering why your service isn't working before blaming the directory and calling in the experts because one error message happens to include the word "logon".