Here's one I haven't seen discussed anywhere. We bought a nice area rug on sale. It looked really good on the hardwood floor in our living room. Over time I noticed a sulpher-ish smell in the livng room. I unfairly began to suspect our dogs.
Eventually I got down on my hands and knees and sniffed the rug. Sure enough, the oder was in our new rug - Gosh darned dogs!
We rented a rug cleaner and gave it the once over. The rug was certainly clean. It still smelled. We suspected something in the manufacture of our rug, perhaps a dye? We thought as time went on maybe it would go away.
Getting impatient, we sent the rugh of to a rug cleaner in Saint Paul. When we went to pick it up he gave us a little nugget of information. Heat is used in the manufacture of some rugs. If the rug gets too hot synthetic materials in the rug backing actually get burnt. You can't see it, but you can smell it. He told us we could never wash the smell out. Sometimes, that's the price you pay for a bargain.