I cannot believe this hasn't been on the tracker before.

Tuna comes in a cylindrical can, i.e.:
https://www.google.co.uk/search?hl=en&authuser=0&site=imghp&tbm=isch&source=hp&biw=1680&bih=925&q=tuna+can&oq=tuna+can&gs_l=img.3..0l10.6036.6969.0.7234.8.8.0.0.0.0.66.420.8.8.0.msedr...0...1ac.1.61.img..0.8.419.Dm3qP-9NEpw

Sardines come in a rectangular can with rounded corners, i.e.:
https://www.google.co.uk/search?hl=en&authuser=0&site=imghp&tbm=isch&source=hp&biw=1680&bih=925&q=tuna+can&oq=tuna+can&gs_l=img.3..0l10.6036.6969.0.7234.8.8.0.0.0.0.66.420.8.8.0.msedr...0...1ac.1.61.img..0.8.419.Dm3qP-9NEpw#hl=en&authuser=0&tbm=isch&q=sardines+can

This has been an issue since day 1, this puts my OCD into overdrive!