Better check

src/main/java/refinedstorage/inventory/BasicItemHandler.java

@@ -21,23 +21,19 @@ public class BasicItemHandler extends ItemStackHandler {
 
     @Override
     public ItemStack insertItem(int slot, ItemStack stack, boolean simulate) {
-        boolean mayInsert = validators.length > 0 ? false : true;
-
+        if (validators.length > 0) {
             for (IItemValidator validator : validators) {
                 if (validator.valid(stack)) {
-                mayInsert = true;
-
-                break;
-            }
-        }
-
-        if (mayInsert) {
                     return super.insertItem(slot, stack, simulate);
                 }
+            }
 
             return stack;
         }
 
+        return super.insertItem(slot, stack, simulate);
+    }
+
     @Override
     protected void onContentsChanged(int slot) {
         super.onContentsChanged(slot);