Actually, that may be the case in one of mine.
It this one (a RED one), I have PureSmoker Firecracker... a cinnamon juice and, so far the mouthpiece has been... well, it looks like, melting. Was getting wider where the mouthpiece touches the silicone cap... to the point where I was having trouble getting it in and out... and so I filed it down and now it is working much better... but, again, this one appears to be fogging up and it just might be some corrosion on the tube.
Will have to keep an eye on this one.... at least now I know where I can get replacement tubes and mouthpieces at a relatively good price (DiscountVapers).
Also, I am relatively new to using these and, was wondering how I know if an atomizer needs to be replaced and, now I know. It you start getting burnt juice from one where you were not getting burnt juice, it means the wick has gotten clogged to the point where it isn't getting enough juice through so what does get through absorbs that much more heat, to the point where it starts burning. You can also get this if the juice in the tube gets real low, but in that case it is just a matter of putting in more. On this other one, I have a Cafe Mocha which was my worst case in clogging up atties and boge cartos, I can try cleaning the atty but otherwise, time to put in a new one. Just made it through one 6 ML bottle before it started burning and, to be honest that is pretty much what I expected with the Cafe Mocha. This one is thus gonna be more expensive to vape than using boges, but otherwise if my other juices make it through at least 12 ML before I need to replace, then it is cheaper than boges, and that appears to be the case.