Here’s the other half of that list that I started in (this post). These are the class of consonantal adjectives that tend not to operate as quasi-nouns, and therefore tend to take -i in the ablative, rather than -e.
āmēns, āmentis, frantic, crazed
anceps, ancipis, double, doubtful
concors, concordis, agreed, joint
dēgener, dēgeneris, low-born, weak
hebes, hebetis, dull, blunt
ingēns, ingentis, huge, vast
inops, inopis, needy, helpless
memor, memoris, mindful of
pār, paris, alike, equal to
perpes, perpetis, lasting
praeceps, praecipitis, headlong
praepes, praepitis, nimble, winged
teres, teretis, smooth
Sorry for the relative obscurity of these last two posts. The thing is, they cover a few footnotes in Allen and Greenough that I feel should be out there on the Internet, preferably with a basic entry format and definition.
Consider section 121a4, which lists a variety of consonant stem adjectives that do not take -i in the ablative singular. They are completely regular, and the entry is really there only to keep you from having second guesses.
I’ll list them here so they get some web mileage, despite not being especially interesting, however rare:
caeles, caelitis relating to the heavens or their Gods
compos, compotis possessing control of
dēses, dēsidis lazy
dīves, dīvitis wealthy
hospes, hospitis amicable, relating to guest-friendship
particeps, participis participating in
praepes, praepitis nimble, winged
pauper, pauperis poor, destitute
prīnceps, prīncipis princely, noble
sōspes, sospitis safe and sound
superstes, superstitis surviving
As someone pointed out in a comment, the general but non-binding idea is that those adjectives which most often operate as nouns (like these) take the ablative in -e, whereas those that are properly adjectival take the ablative in -i. You find hints of this throughout the consonantal and i-stem entries in Allen and Greenough (see 121a1-2), but they make no effort to propagate it as a formal rule.