On second (and following) paste operations empty rows are added when they are not necessary.

How to reproduce:
1) Create table:
CREATE TABLE IF NOT EXISTS `bug_demo` (
  `c1` int(11) DEFAULT NULL,
  `c2` int(11) DEFAULT NULL,
  `c3` int(11) DEFAULT NULL,
  `c4` int(11) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

2) Navigate to newly created table and choose "View Data...". Empty table is shown.

3) Open spreadsheet application of your choice (I used libreoffice), create spreadsheet with following data:
   A   B
1  11  12
2  21  22

4) Select A1:B2 and copy to clipboard

5) In netbeans press "Insert Record(s)", put cursor in column C1 of the first row. 

6) Paste. Notice that window now contains two rows.

7) Put cursor into column C3 of the first row. Paste again.

Notice that window already contains three rows (BUG). Expected result -- only two rows.

8) Put cursor into column C2 of the first row and paste yet another time.

Notice that windows already contains four rows (BUG). Expected result -- only two rows.

If more than two rows are pasted, even more unnecessary rows are added.  If one has to fill wide table and paste just a handful of columns here and there, he will need to remove empty lines at bottom before submitting to server.