I went through a period of coughing up tar, though it tailed off quite quickly (I had a bad flu just before I stopped smoking though so I probably coughed up quite a lot during that time without really noticing it).
Now 4 months into vaping and stopping tobacco completely I very rarely cough up any tar.
I have been noticing since mixing my own juices recently that when I vape menthol and mint flavours (particularly the menthol), after a heavy vaping session, I'll cough up a little clear flegm with little specs of brown tar in it.
Someone on another forum suggested the menthol might be having an astringent effect causing this.
I don't think the tar is coming from the liquid itself as menthol seems one of the cleaner flavours on the atomizer and wick (particularly compared to some of the others I've used that gunk it up worse but don't make me cough up anything), so I'm assuming it's helping me to clean my lungs out in some way.
Its a pity I need to be in the right mood to enjoy methol, but I'm trying to have a tank of it each week regardless, on the off-chance that it's doing me some good.
It might just be me though because I tried searching to see if this was a known effect and couldn't see anyone else mentioning it.
Although my nose is getting a bit sore - as soon as I pick , its replaced by another 
