Top 2 IMHO. 1. Cleaning upholstery with little or no training and 2. Cleaning any area rug you come across with little or no training. Both could be potential disasters. Some find this out the hard way every year I strongly suspect, yet the industry seems to ignore it from what I can tell. Believe me, if you ruin thousands to 10's of thousands of dollars of your customer's belongings they WILL NOT just blow it off. Oh, and not to mention, your insurance more than likely will leave you hanging in the wind. I can see where trying to do flood work with no training could be number 3 if not equal to the other 2 and possibly far worse. The moral of this story is to get some training or you might live to regret the day you were born. I would also learn to say NO to some things and not try to be the hero every time. Hero's can go to minus zero's in a heartbeat. Oh, and in 2023 there is no excuse anyone can come up with not to get all the training they need, and frankly, a lot of it is totally free and widely available now.
As an example, very early on in my career, about 35 years ago, I foolishly attempted to clean some velvet furniture and really didn't know what I was doing. If not for another seasoned cleaner correcting this potential several thousand dollar mistake, it could have been the end of my business and I might be flippin burgers now or maybe diggin ditches!!! I also saw where another cleaner in town just a few years ago cleaned a bad bleeder wool rug in place in a high-end customer's home. Let's just say several thousands of dollars in damage was done when the rug bled into the carpet totally ruining it. He's quite fortunate the customer didn't hold him accountable!!!