It appears that if you add a text field or text area with an unlimited number of values then the code in 6.x-2.9 is adding an 'A' into the next available value every time the form is submitted.

Tracking this through it seems that the 'A' being added is from the Add another item AHAH button.

I found this by outputting the various components in the content_storage function in content.module. This is specifically lines 1068 to 1077. If you output the values in $item you can see that the Add another value is shown here and if you output the $record before it is inserted it shows the relevant DB insert.

I added a quick and dirty fix by replacing:

foreach ($node->$field['field_name'] as $delta => $item) {

with

foreach ($node->$field['field_name'] as $delta => $item) if($item!='Add another item') {

But know this is a bad fix. For starters the value of 'Add another item' could be changed and more likely the button value should not be getting into this function anyway.