I'll still occasionally let customers do this if I'm feeling nice, but now I let them know ahead of time (and have a clause in my contract) that if something goes awry with their materials and/or subs, it's on them to clean up the mess.

It's messy and complicated.  

I have seen CPAs charge extra if the bookkeeping is messy.

Its messy as recording is allowed in public areas, but if there is a "reasonable expectation of privacy" - say its in the back of the house and unless you trespass on the neighbors property you wouldn't see it I think its reasonable to suggest its not legal to record in this area.

It can turn into a mess.

Real estate is a business and it can get very messy getting your family involved with your business. 

Other potential sources are executive rentals, where the home is part of a corporate compensation package (I have one rented that way now), and a messy divorce/chapter 13 bankruptcy where the client must disclose everything as part of the bankruptcy. 

I really like that messy car advice!

Plumbing is lucrative but requires more messy work.

@Nick Mess I just sent you a request and would love to talk to you soon.