Object is invalid or no longer set. (Error 3420)

[izen]

Hi guys !

I have got a problem about adding new items iin the combo box
I am developing a recipe card database
In recipe table has relatioinship with IngredientRecipe
and Ingredient table also has relationship with IngredientRecipe

<Recipe table>
RecipeID
RecipeName
CookingTime
Cost
Method
<Ingredient table>
IngrdientID
IngredeintName
Cost
Unit
<IngredientRecipe>
IngredientRecipeID
RecipeID
IngredientID

All of ingredients will show in combo box in recipe form which is ingredientRecipe subform in recipe table

Begore that It worked fine I can add ingredients and the ingredients will show in combo box and it also be calculate the costing

but just mins ago, I add a new ingredient it did show in the combo box but the calculation is showed error and I have got the error messger box
that "Object is invalid or no longer set. (Error 3420)"

What happens in my database why before this it worked well not problems?
I didnt do anything I believe

hope u guys help me pls

=====================================

I went back to back up file and try to put the new ingredients
it works now - -? so weired !!
However I am afriad of it will happen again
So I want to know the reason

thanks for reading guys