The Horizon Effect, also known as the horizon problem, is caused by the depth limitation of the search algorithm.
When evaluating a large game tree using techniques such as Minimax with Alpha-Beta pruning, search depth is limited for feasibility reasons.
Greedy algorithms tend to suffer from the horizon effect.
The horizon effect can be mitigated by extending the search algorithm with a quiescence search.
Rewriting the evaluation function for leaf nodes and/or analyzing more nodes will solve many horizon effect problems.