Question: I was fighting a Draugr Deathlord in a dungeon (where they are mostly found), and I'd been disarmed. I used unrelenting force to shout him onto a ledge. I took the opportunity to heal myself with some Magicka, and I realized that it was not depleting despite my having used the spell. The Draugr was stuck on the ledge for some reason, so I bombarded him with flames until he died, still having full Magicka despite having used flames.

Has anyone else experienced this? If so, have they figured out how to replicate it? I know it's a glitch, but some glitches can be enacted on purpose.

Answer: 
It is possible to reach 100% magic cost reduction with "Fortify" enchantments for up to two magic types. You need enchanting skill 100 with the final perk to do this.  

For only one magic type you can reach 100% reduction via enchantments by using "Fortify Enchantment" potions to max out you skill.

What I think might have happened here is that you already had some fortify enchants on your gear (not necessarily enchanted by you, there is a lot of gear that already has that) and use of a potion pushed you over the edge for 100% cost reduction.

Another option would be that your mana regeneration rate has become high enough to offset the cost of you heal and flames almost immediately, making it seem like no mana was used. This could also be achieved by a mix of enchants and potions.

A third option would be a combination of the first two. Strong spell cost reduction combined with fast mana regeneration could also make it seem like there was no mana consumed.
My guess would be maybe you have an extremely high magic stat or maybe it just bugged out and didn't show the magic bar as moving. I've never heard off that glitch before. I wish you good luck friend, Talos be with you.