When table rows have a mixed <th> and <td> tags, the last closing </th> is placed at the end of the row code <tr> ... (here)</tr> instead of closing the tag as it should.

See the two examples below:

    || Heading1 || Heading2 | Heading3 |  Heading4 
    |  R1 C1    |  R1 C2    | R1 C3     |  R1 C4 
    |  R2 C1    |  R2 C2    | R2 C3     |  R2 C4 
    || R3 C1    |  R3 C2    | R3 C3     |  R3 C4 

<table class="wikitable" border="1">
<tr class="odd"><th> Heading1 </th><th> Heading2 <td> Heading3 </td><td>  Heading4 </td></th></tr>

<tr><td>  R1 C1    </td><td>  R1 C2    </td><td> R1 C3     </td><td>  R1 C4 </td></tr>

<tr class="odd"><td>  R2 C1    </td><td>  R2 C2    </td><td> R2 C3     </td><td>  R2 C4 </td></tr>

<tr><th> R3 C1    <td>  R3 C2    </td><td> R3 C3     </td><td>  R3 C4 </td></th></tr>

</table>

Already fixed in 2.5.30. Duplicates BugTableHeaderNotXHMTLCompliant.