Mistakes happen and sometimes you need to be able to delete something you already pushed to the git remote. Deleting a branch is pretty straightforward, but deleting a tag is slightly different. Here is an example of deleting a tag v1.2.3:
Recently I came back from holiday and decided to import all my holiday snaps into the OSX photos application from my Nikon D3000 DSLR. Whilst I was away I had been taking some photos on my iPhone 6 which had already synced to my mac at home via iCloud. On completing the import I noticed that the DSLR photos were out of sync with my iPhone photos. At this point I realised I had forgotten to change the timezone!