1. Create a table: [ Foo | <empty> ]
2. Select the whole cell "Foo". This puts a single cell wrapped in a table in the clipboard
3. Select the empty cell. Pasting now would trigger the table-into-tableSelection special case, and just overwrite the empty cell with "Foo": [ Foo | Foo ]
4. Instead, double click to place the cursor in the cell, then paste
5. Because the context is now a paragraph, we paste a full table containing one cell, "Foo", so you have a nested table: [ Foo | [ Foo ] ]. It is more likely in this situation that the user wanted [ Foo | Foo ] instead.

Examples in the wild:

• https://office.wikimedia.org/w/index.php?title=Travel/upcoming_group_travel/All_Hands_2019_Manifests&diff=244979&oldid=244964&diffmode=source
• https://office.wikimedia.org/w/index.php?title=Travel/upcoming_group_travel/All_Hands_2019_Manifests&diff=244989&oldid=244980&diffmode=source