An empty slot in Rack (front/back) view takes up less space than an occupied slot.
The alignment of the front and back column gets out of order if there are more empty slots in one of these columns.

I changed line 632 of the "/idoit/src/tools/js/scripts.js" file to fix this problem.

el.insert(new Element('tr', {
    className: css,
    'data-slotnum': i
}).update(new Element('td').update(num + ' ')).insert(new Element('td', {
    className: 'slot'
}).update( ')).insert(new Element('td').update(' ' + num)));
if (rowspan > 0) {
    el.down('tr:last').down('td', 1).remove();
    rowspan--;
}

el.insert(new Element('tr', {
    className: css,
    'data-slotnum': i
}).update(new Element('td').update(num + ' ')).insert(new Element('td', {
    className: 'slot'
}).update(new Element('div').setStyle({
    height: '19px'
}))).insert(new Element('td').update(' ' + num)));
if (rowspan > 0) {
    el.down('tr:last').down('td', 1).remove();
    rowspan--;
}

This bug is present in the pro version.

The field
Administration->Global settings->Object type configuration->Infrastructure->Power distribution unit->"Icon"
contains the value "pdu.gif".

This file does not exist. That messes up the layout of Rack (front/back) view.

I changed the value of the "Icon" field to "images/icons/silk/lightning.png" to fix the problem.

This bug is present in the pro version.