Weird database error when posting entry

I’m using this build of EE2: 20100121

I’m making a gallery fieldtype, which is an unordered list. Each list item has two file-placeholders(a thumbnail and an image) which lets you select two images. There’s also a caption input field.

I got rid of the error by modifying my copied ft.file code: <pre><code>

    $hidden      = form_hidden($this->field_name/*.'_hidden'*/, $filename);
    $upload   = form_upload($this->field_name, $filename);
    $dropdown = form_dropdown($this->field_name/*.'_directory'*/, $upload_dirs, $filedir);[/code]

I got rid of the _hidden and _directory ending of two form elements and then it worked. Which I find a bit odd having to do since it’s working fine with ft.file…

The changes I did when it started acting up was changing a form_input() into plain html, so I could have more control over the markup generated. I changed a few other small things and when I saw it generated trouble I tried undoing my changes. But it didn’t help

Any idea of where I should begin looking?