When you click the share button under a post, a small popup will appear.
There is a link in a text box to the post that you would like to share.
The problem I've encountered is that I accidentally typed into the link text box, which totally ruined the link, forcing me to close the popup and re-open it.
I do not see why anyone would want to modify the link in any way, since modifying it would completely ruin the purpose of the link, as it might be an invalid url or a link to a different post. I suggest that the text box should not be editable.
I'm not a good programmer, but I found a link to a possible solution here.