Ok here's by far the worst one I've ever treated:
Now, I was a tech in a major-ish city, and with that comes neighborhoods with the standard good and bad areas. The reason I say this, is because it seems like the health department has never heard of these mom and pop places. I swear I'm not racist, but this is just how things were. The Chinese and Mexican restaurants were filled with a grease-like substance. I mean, it coats the walls. The longer the establishment has been operating, the thicker the stuff on the walls is. I'm not talking about chains, mind you; they at least have some sort of cleaning schedule (not to say they're immune from infection, it's just...less). This is of course mainly noticeable in the kitchen area, so if you can somehow sneak into one, run your finger on the walls and you'll see what I'm talking about.
There's a Mexican food market in a bad part of town that has just signed a new pest control contract with us. I arrive there, and no one speaks a word of English. From high school, the best I can say is "yo hablo poco espanol" (high school also taught me Spanish swear words, but those weren't applicable on the job - one time I said "el cucaracha es muerte" and got a funny look). So anyway, the people there point at the walls, and it's obvious what I need to do. I need to burn the motherfucking place down. Unfortunately, that kind of treatment is not allowed by our branch.
Let me point out a tip about cockroaches: they like darkness, dampness, and heat. If you see a roach in the light, more than likely it can't find a hiding place - meaning that all the hiding places are already full of cockroaches. This place had roaches scattered all over the walls (they like to hang out around the top edges of rooms, and congregate into groups). One of the best prevention tips is to keep food in sealed containers, and to make sure there are no water leaks anywhere. There's a reason I mention this. Also, a common theme I see with (cheaper) Mexican and Chinese places is that they build furniture-things with these massive gaps at the joints. Whenever you visit a restaurant, look at where the wood meets, and you'll see what I'm talking about. You'll also spot roaches in there, too.
Ok, this is a Mexican market. It's in a strip-mall, so it's likely that any infestation has already reached the other neighboring stores. It's literally impossible to stop this infection, but I won't be told I didn't try. They have exposed food lying on counters. I'm talking meat. They leave it sitting there all day, and overnight, exposed. In the back of the store is a very small bathroom with a toilet, and water dripping from some random spot in the wall, with water on the floor. Oh yeah, all the walls in the store were half-finished, meaning that the front of the store had sheetrock, but there was nothing behind the walls (except the normal side walls and back of the store). There is a drop-ceiling, with about a foot of space between that and the real ceiling. So we're talking hiding spots everywhere.
They let me into the store, and expect me to treat everything like it is. The chemical we used at the time for roaches was on-contact nasty stuff. We had to wear respirators and gloves, since any contact with skin was bad. Let me say it would be illegal for me to treat around exposed food and water sources. I turn around and try my best to tell them that food needs to be covered and stored...fast forward an hour or so, and everything is finally set - food has been moved into plastic bins (lol), and water has been cleaned up in the bathroom. I can finally get to work...mind you, the store closed at 11pm. The employees wait out front for me to finish since they have to lock the door when I'm done.
So yeah, I find where the main counter meets the wall, and take a flashlight to the crack. Queue the part where several roaches freak out and try to escape the light, which consists of running out into the exposed light. Yep, this is where I'll start. I shoot into the crevice, and flood out an innumerable amount of the little bastards (oh yeah, another thing I forgot to mention - we needed to tuck our pants into our boots and tape them to prevent roaches getting in). There's roaches crawling all over my legs almost immediately after I spray just this one area. Spraying that one crevice also flushed out a black stream of roach-shit - normally roach droppings are around the size of a grain of sand - this was...black colored pest control fluid.
From there, I go around the perimeter of the store, hitting the edge of the floor and ceiling. The drop-ceiling part needed pyrethrin dust (since it was a hollow space), so I needed to put a puff up in there at every ceiling tile. By this time, the walls were literally moving. Roaches were covering all of the plastic sheets covering the store's product (also dropping from the ceiling). I go behind the fake-ish walls, and by this time, they are nothing but moving masses of black. Normal treatment is just perimeter, but I switch the sprayer to "fan", and just crop-dust the walls. For some reason, the bathroom didn't have any roaches in it by the time I got that far (besides the ones I brought in that were crawling on my clothes). Also yeah, I had to constantly flick them off my neck and face. Thank goodness they don't bite. Also, every step I was taking had at least a "crunch" or two.
I end the treatment, and go out the front door. The lady sees that I'm finished, and walks straight in the store, all the way to the back, and turns off the lights. She then walks through the store through the front door and locks it. Even though she has just walked through the seventh level of Hell, she acts like it's just a standard stroll in the park.
They called me back the next night...its impossible to explain that these treatments need time to work, but meh, it's a good excuse to survey the damage. Dead roaches have been swept into piles along the walls. Food was back on the counters, still exposed, with dead roaches lying on the other tables and shelves in the store. The back area wasn't even bothered with, so it was just a carpet of crunchiness. The workers had moved in stock boxes into the back, and apparently just stacked them on the roach-carpet. I opened one of the boxes, and sure enough, it was infested. Welp, I tell them that they need to put the food away properly, and then do a light perimeter treatment...even off that, roaches were still crawling out of the cracks. I finish up without any other issues.
A few days pass, and I'm meeting with my boss. He tells me that they had called the next night, and that he personally went out there to see what was going on. He could tell I treated everything, and noted that the roaches were still left all over the place, without proper cleaning. He said he got the hell out of there, laughed, and called the Health Department.
I drove by there a few days later out of curiosity and they were closed.