Well-intended people often do bad things to their cars - without realizing it, or understanding what the possible consequences could be. So, which is worse?
* Under or over-inflating tires?
Excessive air pressure in your tires will result in a harsher ride - as well as faster (and uneven) tread wear. You'll be paying more for tires - more often. Handling and braking performance may be negatively affected, too.
On the other hand, under-inflated tires are more insidious because people often don't know - or notice - that their tires are low on air - mainly as a result of benign neglect and because tires, air valves (and wheels, if they're aluminum) commonly leak air. A tire usually has to be really low on air - maybe even close to flat - before it's visually obvious. Meanwhile, you're driving around on a tire (or tires) 5, 10 or even 20 psi below the recommended pressure - which can lead to weird handling/poor braking, even a sudden failure due to the heat build-up. The classic case being the Ford Explorer/Firestone tire debacle of the late '90s. People were driving at high speeds, on hot days - on under-inflated tires. Which failed catastrophically - and suddenly - leading to numerous wrecks and deaths.
Point being: Check the air pressure in your tires with a gauge - not just by looking at them - at least every two weeks. Be sure they're all filled to the specified pressure - neither too much nor too little.
* An extra quart in the crankcase? Or down a quart?
Many people take their cars to a quick lube place for oil changes. These places typically use a gun to shoot new oil into the engine - as opposed to pouring it in a quart at a time. It's not uncommon for the result of this to be too much - or too little - oil in the engine. What's the difference?
Running a quart low is less potentially damaging than running a quart over. Reason? If there's more oil in the crankcase than the engine was designed to hold, that oil has to go somewhere. What happens is it gets turned into a foamy froth as the engine's reciprocating parts churn it around. This foamy frothy oil is less able to lubricate vital parts -which can lead to a catastrophic engine failure. Or, the excess oil might get sucked into the combustion chambers, where it gets burned - partially - with the resultant oily residue ruining expensive 02 sensors and catalytic converters.
On the other hand, most engines will not be harmed if run a quart or so low for a little while. Since all internal combustion engines use a certain amount of oil during normal operation, it's expected the level in the crankcase will occasionally be less than "full." The engineers who designed the engine designed in an extra margin of total oil capacity for just this reason.
The key thing is to be sure to check the oil level immediately after every oil change, if you have your oil changed by someone else - and at least every couple of weeks thereafter.