Nothing, just one small oil leak that went away on it's own.
I love this car. I bought this car from my neighbor in 2002. He took great care of it as do I. It runs great, quiet, and looks and feels, and runs like new. Most people can't believe it is 11 years old and has 162,000 miles. I would definetly buy another Volvo when the time comes to buy a new car. No maintenace problems only the regular wear and tear of tires and brakes and oil changes.
I have one and I love it got it at 38000 miles and I trust to go anywhere.
These 850's are less reliable, more expensive to repair/maintain, and more fickle for maintenance than the older rear-wheel driver 4cyl Volvos (240/740/760/940), BUT they are still longer lasting and more reliable than the average car. Just keep up the regular maintenance schedule, change the rear main seal around 100k, and don't drive it like a race car. It'll last for 200k+ miles on the original engine and transmission.
I got my 94 Volvo 850 in 1996. The original owner took great care of it. Last month (Aug 2006) I gifted it to my son who lives in Colorado. He flew to Miami and drove it back to Evergreen without any troubles.
There is a minor air condioning problem, but it should be easy to fix.
The South Florida heat was not good for the plastic hooks that keep the instrument panel in place. So, we have some rattles, but who cares.
Before giving it to my son, I replaced the rear main oil seal and put in a new clutch.
The car has around 200K miles and does not burn oil. There is no rust and otherwise it runs great. That five cyl engine is amazing!