We had a neat 1" hole on the underside of in an exposed 4" cast iron pipe in the basement. The hole was just before a coupling. The pipe was coming from the kitchen upstairs and came down through the floor boards and then bent round and traveled horizontally just above the basement door before heading into the main stack.
Our best guess was that someone had poured drano or some other corrosive chemical down the sink and then not flushed with water and left it for quite a while.The corrosive chemical had accumulated in the dam formed where the coupling met the pipe and eaten away at the pipe.
I had an interesting experience with plumbers trying to get this fixed. The first guy I contacted was a contractor for a big plumbing company. He took a look at it and gave me a written estimate for $650. I looked on yelp and found another, local, plumber. I like to get multiple estimates so I can get an idea of where things are. If the estimates are similar I like to go with local, small businesses.
I had a very, very odd experience with this second guy. He looked over the job and then became very cagey. When I asked for an estimate he said it was hard to estimate how much. He said the access was bad, he would have to cut the floor board, then depending on how things went, perhaps he would need to open up the cabinets in the kitchen (we have built in cabinets) and it could run between 1000 and 1500 dollars.
While he was telling be this he was looking at me very closely, as if he was trying to read my face. I did not like that. My first thought was that he was trying to figure out how much the market (me) would bear. As a condition for my insurance binder I needed a written estimate of the repair costs and so I asked him for it. He became very evasive and refused to put anything in writing.
His next response was very odd. He sided up to me in a conspiratorial air - "Are you trying to buy this place?" He asked. "Are you trying to get some money back from the seller?" I just didn't go there. I said no, it was nothing of the sort, I just wanted to get it fixed. But that immediately put me off him. The implication was that he would inflate the quote to 'help' me out. A dishonest person like that is not to be encouraged.
Then he started to become very aggressive. He got in my face and said "See, I'm looking you right in the eye when I tell you the price. Would I lie to you? If any one tells you a different price he's full of beans" This went on for a while and then I showed him the door. I had taken off from work for this and it turned out to be a wasted half day.
I got the first guy back, he did the job for what he quoted and it was an overall pleasant experience. This was a bigger, national, plumbing company, but I realized that the plumbers were local and were basically contractors. Any how, buying a house made us realize the importance of interviewing and getting to know the contractors who will be doing the work.
As a side note, the plumber fixed the hole by replacing the whole horizontal section of the pipe. He used compression joints (I later found they were called flexible couplings, no-hub/hubless couplings or shielded couplings) and that was the first I had ever seen those. The job was indeed a little tricky: the access to the pipe was hindered by the floor joists and the edge of the foundation. The plumber was good and cut out the pipe sections without damaging the joists or cutting into the floorboards.