Morphological comparison of adjectives ending in "-ic"
Page 267 of Fowler's Dictionary of Modern English Usage reads
Adjectives ending in -ic (comic, rustic, etc.), -ive (active, restive, etc.), and -ous (famous, odious, virtuous, etc.) do not have -er and -est forms except in special circumstances.
What special circumstances is the author referring to?
Secondly, what would such forms be for comic? comicker/est?
One example of the special case would curiouser, an er form of the adjective curious. It is a special case of neologism derived from a work of literature, in this case, Alice in Wonderland.