This might be due to the fact that I'm not a native speaker and I encountered this phrase at a later date, but people saying "it's all but xyz" to mean "it's xyz" really gets on my nerves. I get it, "it's all but complete" means that virtually all the conditions are met for it to be complete, but I find it so annoying for some reason.
"The task is all but impossible" registers as 'it's not impossible, it's everything else: possible', so the fact that it means the opposite of that makes my brain twitch.