We and a lot of others I know do stain removal for insurance companies. How this works is if you get the stain out, usually 75-80% success rate, you usually get to clean the whole carpet. You also get your call out and a stain removal success fee. This makes it a great little earner. If you don't manage to get the stain out you get a flat rate of anything between £20-50. Therefore you don't want to be driving 40 miles if you aren't going to have a good chance of removing the stain. So what happens is when you make contact with the customer you interogate them (in a nice way) and ask them if they have tried to remove it themselves. If they say they have used Vanish or 1001 or any other concoction from under the sink we normally just try and get room dimensions, type of carpet etc. A lot of the time they say they haven't touched it and when you get there what do you find but a big bleach mark where they have obviously knacked it, you feel like strangling them.
In short once the customer has messed with it this makes the job of removal so much harder.
Don't get hung up on stains, you are not a magician. Remember they caused it and as Simon has said if you start to mess about too much the customer ends up starting to blame you. Best to have a go and walk away if it doesn't shift. That way your reputations intact.