avoiding sarcasm
empty collection? or empty collection list? search for string to find:
I don’t know how you got here, but this is a sad place --> "Create a collection by clicking on the star icon while reading a Wikipedia article"

To give user more clarity about what a collection is for:
Blue notfiication overlay:

Would you like to add [page] to a collection? --> Add [page] to a collection of articles you can share with the world!

let user know error had to do with their entry, not the server malfunctioning
When abuse filter or other validation is triggered:
Creating an account:
there was a problem adding the item to your collection -->"There was an issue with the name you entered. Please try something else"

Editing an account:
there was a problem saving the changes-->"There was an issue with your attempted change, please try something else"