I was recently asked by a family member to create a Facebook cover image and profile picture for a shopping center she (as a real estate agent) is promoting to large corporations as an option for their branches.

I created the images and sent it through to her, but i did not check the quality or if the sizes were correct. It was only after two weeks, that she messaged me and told me the images were being cut off, that I realised the sizes were incorrect.

A rookie mistake, luckily I quickly changed it. 

Lesson learnt - always test and triple check everything.