The CookMug Lids has demonstrated shrinkage after use, resulting in a loose fit with the CookMug. We are switching material immediately to a polymer with increased heat resistance. Under our Warranty, we will ship replacement lid(s) to existing customers automatically and free-of-charge. We expect to ship the new lids next week.
If you plan to use the CookMug Lid in the meantime, we recommend that you use it only after bringing your CookMug to boil, and first check that the fit is secure between the CookMug and the CookMug Lid before drinking from the CookMug.
We apologize for any inconvenience as we resolve this issue.