problem:
If the custom attribute properties don't validate, the field is created/altered in the table, but not added to the meta data.

to reproduce:
create a new document template.
add an attribute (type=Long Text, display=HTML)
set the width/height properties to something invalid (like 100%)
click ok.

First you get a validation error, but after correcting the values and clicking ok I get this message:

Kentico.CMS.DataProviderSQL.ExecuteQuery: Query: ALTER TABLE [CMS_HaloLinkbox] ADD [List] ntext NULL : caused exception: Column names in each table must be unique. Column name 'List' in table 'CMS_HaloLinkbox' is specified more than once.

solution:
make these operations atomic/transactional because this very hard to work with.
Now I have to dive in the database and remove the field.