Two of the most popular, or common, causes for penalty strokes are lost golf balls and hitting into an unplayable lie.
Most often these situations both arise when you’ve put your golf ball somewhere in a wooded area. Golf course designers love to put treed areas off to the sides of fairways so be careful when you’re around such conditions. I’ve spent more time than I’d care to admit rooting around the woods looking for lost golf balls. If you can’t find the ball you can go back and hit again and take a penalty.
The unplayable lie is a really good opportunity to strictly follow the rules and be an honest golfer. It’s also a good time to look the other way and cheat a little. Honest golfers will either more the ball two club lengths and take a penalty stroke or go back and hit another shot from their original spot and take the penalty stroke. Dishonest golfers (i.e., everyone I know and play with) will just kick the ball slightly to give it a better lie then hit their shot. The choice is yours to make.