diff --git a/src/core/pool_type.hpp b/src/core/pool_type.hpp
--- a/src/core/pool_type.hpp
+++ b/src/core/pool_type.hpp
@@ -32,7 +32,9 @@ struct Pool {
 	size_t first_free;   ///< No item with index lower than this is free (doesn't say anything about this one!)
 	size_t first_unused; ///< This and all higher indexes are free (doesn't say anything about first_unused-1 !)
 	size_t items;        ///< Number of used indexes (non-NULL)
-
+#ifdef OTTD_ASSERT
+	size_t checked;      ///< Number of items we checked for
+#endif /* OTTD_ASSERT */
 	bool cleaning;       ///< True if cleaning pool (deleting all items)
 
 	Titem **data;        ///< Pointer to array of pointers to Titem
@@ -69,7 +71,11 @@ struct Pool {
 	 */
 	FORCEINLINE bool CanAllocate(size_t n = 1)
 	{
-		return this->items <= Tmax_size - n;
+		bool ret = this->items <= Tmax_size - n;
+#ifdef OTTD_ASSERT
+		this->checked = ret ? n : 0;
+#endif /* OTTD_ASSERT */
+		return ret;
 	}
 
 	/**