I frequently see one of these used when the other is required.

Problem:
The adjectives “historic” and “historical” are not synonyms.

Explanation:
The adjective “historic” means important in history or well known.

The adjective “historical” means characteristic or pertaining to history, belonging to the past, or concerned with history.

Therefore, all historic events are historical, but all historical events are not necessarily historic.

A “historical house-restoration project” would be a project involving the restoration of a house in such a manner as to be concerned with the history of the house.

In contrast, a “historic house-restoration project” would be a house-restoration project that is noteworthy for the techniques used in the project, for the accomplishments of the project, or for something else important about the project that will be remembered in the future.

Solution:
Use “historical” when modifying a noun or noun phrase so as to relate it to history. Use “historic” when modifying a noun or noun phrase so as to relate it to something important in history.