To replicate:

1. Insert a table 5 x 4
2. Merge one or two of the inside cells
3. Try to highlight those merged cells, and the rows or columns around them

Another test:

1. Insert a table that looks like this:
2. Click on the white cells and then try to highlight a column

Highlighting will act as if the cells haven't been merged, and sometimes highlight/unhighlight the columns or rows adjacent to the cursor. Verified on Safari and Firefox.