Anyone who's had the misfortune of cleaning a carpet that's had Woolite or a similar Spray & Vac carpet cleaner on it knows just how quick it can fill your vacuum hoses and fill up your recovery tank. Normal, liquid defoamers are virtually useless in those cases and only something like Chemspec Crystal Defoamer applied liberally to the carpet will save the day. I don't run into severe foaming issues near as much as I used to, but I don't leave home without a powdered defoamer. Never know when you'll need it. Loss of vacuum can occur within seconds to no more than a few minutes. All your lift and cfm virtually disappears. A customer who spot cleans frequently with the wrong type of spotter can cause similar issues. I've also had residues left behind by another carpet cleaner on a few occasions that were pretty severe.