I think of myself as a pretty good diagnostician, but I always leave automatic transmissions to the pros.
So when I bought my niece an '02 4wd Pathfinder, and while driving it around getting it serviced and detailed before her birthday I found the trans slipping pretty badly, I thought I was sunk.
I skipped past my normal methodical diagnostics and took it to a well regarded shop; they serviced the truck and told me the trans needed replacing, but it would survive a short time. Another opinion by another shop confirmed it and, so, damn.
I parked it outside for a month while considering my options, and then decided to try doing a filter and gasket service with Nissan fluid and some juju. It was filthy from the weather, so before starting I took it to a local manual car wash and blasted the heck out of it, and ...even did the undercarriage and under the hood.
You know the rest: while driving it back from the wash, it didn't even come close to slipping, and I drove it around for another hour until the slippage returned.
A thirty cent length of vacuum hose instead of huge money for a tranny.