Damage to furniture!
One of the most over looked procedures by some who claim to be a professional and the DIY’er is not protecting furniture from moisture after cleaning a carpet, why? Because usually with hired/rental machines they are not used in the correct manor with plenty of dry passes to extract maximum water from out of the carpet.
Even if you have the most powerful equipment that the world has ever seen your furniture needs to be left out of the room for 24-48 hours to allow the carpeted area to dry properly, or use furniture tabs or polystyrene blocks that any true professional always has to hand to put under your items.
If this is not done you are either very lucky or you will find that your furniture is damaged from soaking up moisture or vice versa the varnishes from wood can make marks on your carpet.