Things sit on the counter too long. If it's hot food, keep it hot. If it's cold, keep it cold.
Just because it doesn't smell doesn't mean it isn't spoiled. Potentially hazardous things like ground meats, eggs and milk may go bad the quickest and you may not know it. Those are things that bacteria like to live on.
It's egg and milk products that people abuse the most. People leave pumpkin pie out on the counter, but it has eggs and milk in it. It's a custard. Don't leave it out.
When in doubt, throw it out. I seriously stress that when people call me.
The expiration dates on packages are 'best used by' dates. After that, it's not unsafe, but the quality is decreasing every day. I usually recommend no more than a couple of weeks past the expiration date.
