Is the Textbox a Bound one? If you select it and go to Properties - Data does the Control Source Property have the Name of a Field in it?
In order for a product 'number' to use the Format of 12-001, the Field has to be defined as a Text Datatype, not a Number Datatype; the dash/hyphen in the 'number' requires this.
It sounds as if your Field is either Unbound or the Field it's Bound to is defined as a Number.
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The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007