the game completely locks up forcing me to either end the process or have a staring contest with the creeper-faced block I was chiseling. This started happening suddenly, as the chisel was working earlier today. One of the other players on my server says it's been happening for him for a long time.. I wish I could provide more information, but there was no crash report as there was never a crash (i.e. the program froze rather than dumping and exiting). The best I can give you is the game logs, and hopefully you can make some sense of it.
client FML-latest log
server FML-latest log
perhaps worth noting, this is happening on a sponge server. I doubt sponge is actually part of the cause, seeing as it's a client crash, but I've learned it's better to just say so at the start when reporting bugs discovered on my server
Also, if this is a duplicate, sorry. There were several pages when I tried to search for similar issues, and even with the best search parameters I could think of, I was ending up looking through several completely unrelated issues for every remotely related one

Your client log seems to just cut off at a random point. Is this the fml-client-latest.log file?

I truncated it (can't remember why), but could've sworn I updated it to the full version of the log. I guess not. Let me fix that

or were you referring to how it cut off at the bottom? I truncated the top, but the cut off at the bottom is because Java seized up, forcing me to end the process (and thus abruptly end whatever logging was taking place)

full client log
full server log

@tterrag1098 ^

What specifically are you doing to trigger the crash?

I'm opening the chisel UI, grabbing a block from my inventory, and trying to place it in the chisel's target slot. The instant I attempt to place it there, the game freezes

I can't seem to reproduce it in single player myself, which makes me think it had something to do with sponge or maybe I was testing using that stack of stone that had NBT somehow?