Let's say the top row of cells in a table has been merged to put a title, it is
not possible to delete any single column below. 

Reproducible as:
1. create any table (e.g. 3x3)
2. merge cells of first row
3.a right-click in any column > column > select
    -> selects entire table instead of single row
3.b accordingly, right-click in any column > column > delete column
    -> deletes entire table

Of course, there are workarounds to do it like
- deleting the merged row and inserting a new one, or
- splitting the merged cells again,
but that's all but convenient.