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

Problem:
The adjectives “classic” and “classical” are best not treated as synonyms.

Explanation:
The adjective “classic” means of the highest quality or class.

The most common definitions of the adjective “classical” are pertaining to Greek and Roman antiquity, consistent with classicism, and, in music, pertaining to the most enduring types of music.

So a classical song typically is classic, but not all classic songs are classical.

Solution:
Use “classical” when referring to antiquity or music. Use “classic” when referring to something of highest quality.