Sorry, but your problem does not imply a bug in PHP itself.  For a
list of more appropriate places to ask for help using PHP, please
visit http://www.php.net/support.php as this bug system is not the
appropriate forum for asking support questions.  Due to the volume
of reports we can not explain in detail here why your report is not
a bug.  The support channels will be able to provide an explanation
for you.

Thank you for your interest in PHP.

You cannot use arrays or objects as keys. Doing so will result in a warning: Illegal offset type. Check your code.

the OP is complaining that it's not documented, not that it's happening.

Yes, but it's documented in http://www.php.net/types.array which covers the use of an array as a key anywhere in any PHP code. It's not really specific to the unset() function.

There needs to be a way to search documentation for error 
messages like this. Maybe a PHP wiki?

If you enter illegal offset type into the search box top right and select "online documentation" from the drop menu.

The first result in the search is:

Arrays - Manual
A map is a type that maps values to keys. This type is optimized in several ways, so you can ... Doing so will result in a warning: Illegal offset type. ...
php.net/manual/en/language.types.array.php - 21 Feb 2007

Even if you search google for the same query, the relevant PHP page is first ;)

Your code should be:

Reproduce code:
---------------
      foreach ($form['taxonomy'] as $key -> $vid) {
        if (substr($vid, 0, 1) != '#') {
          unset($form['taxonomy'][$key]);
        }
      }

Thank you. I would still like to see a section of the documentation devoted to 
error messages.