A few days ago, I deleted 2 MoveableType categories that did not have any entries in them. The next time I tried to publish an entry, I got this error:

Can't call method "label" without a package or object reference at plugins/sub_cats.pl line 67

It turns out that the Subcategories plugin could not deal with non-continuous numbering of category id's. Deleting the two categories caused gaps at #3, and #11.

I did a quick search and came up with this page:
Comments on SubCategories version 0.2.1

I installed this version, but it did not fix the problem - I continued to get errors. So I went into the database, and manually inserted two new rows in the table mt_category, and set the category_id for them to 3 and 11.

Later on I found that there is an even later version, and will be installing that soon.