If a function tag is created that refers to a function that does not exist, the function will refuse to load. According to the logs, the game crashes, however the game just gets stuck on the "load world" screen and is uncloseable except from the task manager.
From the logfile:
00:46:00 launcher Preparing to launch minecraft client for 17w49b
00:46:00 launcher Checking installations.
00:46:00 launcher Minecraft client 17w49b is ready to start.
00:46:00 launcher Starting!
00:46:01 launcher Using default game log configuration client-1.12.xml (outputs XML)
00:46:02 bmz Setting user: PhilosophioStone
00:46:03 bnc Skipping bad option: lastServer:
00:46:03 bmz LWJGL Version: 3.1.2 build 29
00:46:04 sj Reloading ResourceManager: Default
00:46:05 clr Missing sound for event: minecraft:block.pumpkin.carve
00:46:05 clr Starting up SoundSystem...
00:46:05 clr Initializing No Sound
00:46:05 clr (Silent Mode)
00:46:05 clr OpenAL initialized.
00:46:06 clr Sound engine started
00:46:08 ciq Created: 1024x512 textures-atlas
00:46:10 com.mojang.text2speech.NarratorWindows Narrator library for x64 successfully loaded
00:46:10 net.minecraft.realms.RealmsBridge Realms module missing
00:46:54 aoq Loaded 0 recipes
00:46:54 clh Starting integrated minecraft server version 17w49b
00:46:54 clh Generating keypair
00:46:54 sj Reloading ResourceManager: Default, turing, turing_functions
00:46:55 aoq Loaded 466 recipes
00:46:55 k Loaded 509 advancements
00:46:56 nk Loaded 14 custom command functions
00:46:56 net.minecraft.server.MinecraftServer Encountered an unexpected exception
com.google.gson.JsonParseException: Unknown value 'tmf:'
00:46:56 net.minecraft.server.MinecraftServer This crash report has been saved to: C:\Users\elyis\AppData\Roaming\.minecraft\profiles\experimental\crash-reports\crash-2017-12-08_00.46.56-server.txt
00:46:56 net.minecraft.server.MinecraftServer Stopping server
00:46:56 net.minecraft.server.MinecraftServer Saving players
00:46:56 net.minecraft.server.MinecraftServer Saving worlds