My current dog is the only one picked as a puppy, I've always had older rescue dogs before.
The breed was carefully picked (and I still don't think we picked the wrong breed as the breed traits he has suit me very well).
The breeder was carefully picked - though because of the issues since and their responses that possibly was a mistake as they weren't quite as good as they seemed.
We're experienced owners, he was carefully socialised, attended 3 lots of classes and knows more commands than my DC do.
But what I've ended up with is a dog with multiple health issues, which have cost in 2 years more than £10 000 in vet's bills and behavioural issues with strangers and dogs so severe that he has to be muzzled in public, can't be offlead anywhere where other people might be and has to be separated from visitors.
If we ever become unable to manage him then he will have to be put to sleep as there are no other viable options for him.
Despite all that, I still adore him because with all his issues he's a lovely dog, inside, with just us here...but he sure as hell isn't convenient despite all my best efforts to raise a well adjusted, well behaved dog.
Not many people have quite that much of a problem, but, it can happen.
Mostly it's small things, my last dog was a bin raider, the one before that we never quite cracked a reliable recall and things like having to take a dog into consideration when planning a day out.
It's a bit like having children - it's not convenient at all, loads of responsibility and there's always the possibility that you end up with a lot more than you bargained for, but at the same time, worth it.