Consider the following example:

public abstract class Base<T extends Event> implements Listener {
    abstract void method(T event);
}

public class Derived extends Base<BlockPlaceEvent> {
    @EventHandler
    void onEvent(BlockPlaceEvent event) {}
}

If you examine Derived with javap, you discover that it actually has two methods.

Compiled from "Derived.java"
public class Derived extends Base<org.bukkit.event.block.BlockPlaceEvent> {
  public Derived();
  void onEvent(org.bukkit.event.block.BlockPlaceEvent);
  void onEvent(org.bukkit.event.Event);
}

In Java 7u79 and earlier, the @EventHandler annotation only exists on the void onEvent(org.bukkit.event.block.BlockPlaceEvent) method. However, a change was made to the JDK (see https://bugs.openjdk.java.net/browse/JDK-6695379) to copy the annotation to the overridden method, that being void onEvent(org.bukkit.event.Event).

Bukkit reflectively looks for the getHandlerList method for any class in the parameter list of a function annotated with @EventHandler. In earlier versions of Java 7, it would only notice the void onEvent(BlockPlaceEvent) version and successfully register the class. In code compiled with the above mentioned versions, @EventHandler is copied to the void onEvent(Event) method, and then an exception is thrown when Bukkit can't find the getHandlerList method.