I couldn't say it's the same thing as you experienced but I have experienced getting nasty yellow burning residue on my lips from hitting a GVG. I always attributed it to the leftover burnt DMT left in the pipe and some friends suggested that it was because I was tilting the pipe slightly downwards. I've also experienced something along the lines of what you talk about when feeling like something is stuck in your throat when I hit some burnt DMT 'resin' from the GVG. I felt like my throat was closing and it scared the shit out of me. For a good 20 minutes afterward my throat felt awful, sore, and like something was clogging it up. I also suspected leftover solvent? Some pulls were done with naphtha, but most with xylene.