pfSense

If filterdns resolves a record to an IP that exists in a table, and then later switches away, the hardcoded IP will be lost.

A practical example, have an alias with this:
1.2.3.4
myhost.dyndns.org

Initially, myhost.dyndns.org would resolve to something else, say 5.6.7.8. The table now contains:
1.2.3.4
5.6.7.8

Then the myhost.dyndns.org user visits the site with 1.2.3.4, which filterdns picks up. The table now contains:
1.2.3.4

The myhost.dyndns.org user leaves again, and myhost.dyndns.org resolves to 5.6.7.8. The table now contains:
5.6.7.8

The original hardcoded 1.2.3.4 is lost because filterdns removed it during the last switch.