Another challenge is in dealing with odors. I recently had a job in a home where the new residents could smell that the previous tenant's dog had not always made it outside to relieve himself. But narrowing down where these accidents had occurred can be quite a challenge. It took some time, but with the help of a moisture meter I was able to find and mark the areas in the carpet that needed special attention and flush them out with the use of a flash extractor.
But perhaps one of the most interesting situations I've run across to date was in a seasonal home last week. The owners arrived to find an "expired" duck in their home. Apparently some guests from the previous month had left a door open while loading their car and the duck had entered. The guests then locked up the house unwittingly trapping the poor duck inside. The surprise awaiting the owners when they showed up was, well, fowl. Using an enzyme based cleaner I was able to remove the stains from the droppings the duck had left as he wandered the home searching for a way out. The story didn't end well for Mr Duck, but at least the 'bill' (pun intended) was a lot less than the cost to replace all of the carpeting.