I am new in a company, I didn't know every staff has his/her own mugs, although there is a message on the cupboard door, so I used different mugs every day. A lady told me this at lunch time today. I want to write a sorry card, this is draft: Hi friends, I am the person who used your mugs as spare ones, thank you for your generous and kindness. I am so sorry for my carelessness. This is my little treat, please feel free.