A random thing I just remembered:

https://github.com/id-Software/Quake-III-Arena/blob/dbe4ddb1...

They implemented adaptive huffman coding but ironically only use it during static initialization, feeding it with static data (see the defn of msg_hData a few lines up.) Note that the only calls to Huff_addRef are in the init, and notice how it gets called - the loop over msg_hData[i]. I found that pretty funny. I'm guessing there was some plans to use it adaptively but it's not easy/obvious how to keep it in sync reliably...