In the northern tier of the country, many play what is called "the leaf rule." Around mid-September the leaves start to fall and at times there can be very large collections of leaves on the course. Hitting a ball into these areas often results in a problem. Either you lose the ball or, in searching for the ball it is accidently moved or stepped upon.
Variations on the leaf rule abound. A friend allows one to take a drop in the area in which the ball was lost with a 1 stroke penalty. Others require the ball to most likely be in the fairway before invoking the leaf rule. Most of the time the "rule" allows one to drop in the area where the ball is most likely to be without penalty, provided the area is a part of the course (i.e. grass that is mown - fairway or rough).
If scoring for handicap purposes, one probably should not use the leaf rule or consider the hole as incomplete and record what one's most likely score would have been. If you have money riding on the match, make sure everyone is in agreement about whether the rule is to be used and how (I almost had a bad experience with this recently but I found my ball

).