Lotion

When the sun bakes down on bottles with anything inside, it creates vapors that get trapped in the bottle. Without the bottle being open, the vapors have no where to go so they just build up until the bottle is open. Sometimes you open the bottle and sometimes the vapor does. If the lotion explodes, it leaves oil stains where it landed. It's just the worst.

Hand sanitizer

The same concept of vapors applies here. This time though, your car smells like sanitizer and the sanitizer somehow seems more liquidized than water on a really hot day.

Any bottles really, even water bottles

If I leave a drink in my car for an more than a day, its gone. It just doesn't taste the same after its been constantly changing temperature with the weather. This is more of a recommendation so if it doesn't bother you then keep doing what you're doing.

Perfume

I had a $16 bottle of perfume that had been discontinued in my car and it was fine until one particularly hot day. I had worked a nine hour shift, got in my car and reached to grab it; only to find out that it was completely evaporated. Seriously, I hadn't even used half of it. So if you choose to leave perfume in your car, make sure its a perfume you're indifferent about.

Food

I have left a lot of food in my car in the four years I have been driving. Sometimes its fast food or left overs I decided to take to work that I just forgot about. When I get back to my car, there's either an intense smell of food or hot garbage with food. Please do not let food just sit in your car when its hot, the consequences are not pleasant.

Trash

If there's enough trash in your car, it could block the air vents and/or give off a very foul odor. Just throw it away as soon as you get to your destination.

Gum

I have had gum melt in my cup holder and that is not easy to clean. If you chew it when it's hot, it's somehow stickier.