Falsecarding
In general, the partner will gain more from a player's signals than declarer, so it is worthwhile to signal honestly most of the time. However, since declarer can see your signals as well, a player who gains a reputation for always giving accurate count, for example, may find information given away by their signals used to their disadvantage. So it is common practice to throw in a misleading signal now and then, hopefully when it won't matter to partner.
Read more about this topic: Signal (bridge)