If you smoke from tobacco from a marijuana pipe can you fail a mouth swab?

I would assume so, yes. When you smoke from a pipe, residue from the marijuana or tobacco (or whatever you're smoking) builds up in the pipe, in the form of a thick black tar-like substance called "resin." That is the same stuff that is also building up in your lungs...

Anyway. Marijuana smokers will often scrape this resin out of their pipes when it builds up, and smoke it, thus making their weed supply go farther and saving themselves some money.

Often it is possible to just smoke the pipe "dry," as in, with nothing in it. Since the resin often builds up in the bowl of the pipe (the place where you put the marijuana or tobacco), you can often just light the pipe dry and get a few hits.

So, if you put tobacco in a pipe that had weed in it, you will end up smoking a little bit of the resin from the weed, every time you light the pipe. So there is a pretty good chance you will fail a drug test afterwards.