I started my commercial cleaning firm some 40 years ago, right ofu of college. Like many, I had a lean few years, and took most any odd piece of business that came my way - including vacant apartment clean-ups.
Bright and early one morning, I took a call from an occasional apartment manager client; he said he needed a bit of carpet cleaning.
It seems that one of his elderly tenants (call him Harry) had fallen asleep while somking, set fire to his pajamas, and staggered, all a-flame, into the living room, and collapsed and experied right there. ("Live by the weed, die by the weed.")
After removing what remained of Harry, my client (the apartment manager) realized that he needed to replace the chared living room carpet. But, being an economical soul, he elected to replace just the burned spot. The carpet vendor did a remarkedly good job matching color and pattern, but the new carpet was clean and bright, the surrounding old carpet a bit dingy. The manager needed us to either clean the old carpet or to "dirty-up" the new, whichever would be the least expensive. It seems that poor Harry's widow still lived in the apartment, and would get upset whenever she entered the living room and saw poor Harry's final outline, right there in the carpet.
I took two lessons from the experience. First, be ready to provide a range of cleaning and service options to the client, and second, carefully screen your clients.
I do qite a bit of carpet cleaning, mostly built into ongoing janitorial contracts for commercial accounts. I also do VCT (Vinyl tile) burninhing and refinishing, and ceramic and stone scrubbing, on the same no extra charge pattern. And we do windows - a lot.
But, I don't do much apartment cleaning any more.