One other field software development is similar to would be dentistry (okay, actually, there are a lot of different things, but this one would make it easier and funnier to relate to).
Imagine one day a patient coming in and asking to treat his tooth.
Imagine one day a customer coming in and asking to do support/consulting stuff on his project.
There are different ways it can go on. You can know this person for years and all the services were bought from your company, so you know pretty damn well what's going on there.
But usually, it would happen that customer is just trying to get a better price, or their previous goto man left the field, or something just didn't work.
And then starts the fun part.
It might be that customer doesn't have any complaints and then you could just let them off with advice of flossing and hoping everything's gonna be okay. Or you just need to do some whitening, and that's it and everybody is happy.
In case there are complaints you would try to ask some questions for the medical history. Rule of thumb is customer knows some stuff s/he was exposed to, but they might
1) confuse difficult (lamost foreign sounding) terms
2) not being fully aware of all the previous decisions made while being treated
And with every question customer might be getting angrier and more impatient, because the problem at hand seems soooo easy and it always took just 15 minutes, but not this time for some reason. It really depends on how thorough you can afford to be and what type of customer you are dealing with. Good professional won't be easily withhold by someone just being angry, as they are doing their best in order to acquire fullest picture so that everyone is on the same page and noone gets to stumble in darkness with hidden surprises everywhere.
You could get a medical history (aka docs).
But there might be a case it wasn't maintained properly ('yeah, this is my card from years ago, but then I had like 4 teeth pulled out while on vacation in Cambodia'). It might be conceived before laser printers were in every clinic, so it's hand written or uses foreign language, uses some cryptic abbreviations you have never heard of and on top of it, few pages are missing and one has a huge coffee stain on it.
You might try to get more info but running some test or taking x-ray and trying to get an idea of what's going on. But there are always hard cases when you cannot easily tell what's going on. People afraid of x-rays, tongue piercing coming up on shots, people overthinking the sensations they get from dentist explorer – all that might lead to a certain degree of false positives.
Worst part of the story is understanding someone messed up years ago but it's only now that patient/customer needs to take drastic measures to prevent even bigger issues that can even pose threat to someone and just redo everything done before.
If you liked this tiny one, take a look at part II.