In an answer to a specific family that was suffering from a medical issue, the Rebbe responded:

בטח שגגו באכילת דעניני שמיטה.

When they received the answer, they asked how to start checking into this, as they are generally careful to only buy products with Mehudar Hechsherim. The Rebbe responded:

עבודת ה ' בשמחה (שפורצת גם גדר דשגיאות מי יבין).

-ליקוטי שיחות חלק ל"ו, ע' 298

In an answer to a specific family that was suffering from a medical issue, the Rebbe responded:

They transgressed unintentionally, by eating food to which Shmitah applies.

When they received the answer, they asked how to start checking into this, as they are generally careful to only buy products with Mehudar Hechsherim. The Rebbe responded:

Serving Hashem with joy (which breaks also the boundary of unintentional transgressions).

(Free Translation)

-Likkutei Sichos 36, page 298