A cross-UPDATE is an UPDATE operation that includes one or more other tables, using a JOIN. This is very useful in order to update one table with a value derived from another table.
Here’s a strange insight that I gained when building a test case where I needed some randomized values. In order to generate random values, you can use the NEWID() function, which creates a uniqueidentifier value for each row. But NEWID() comes with a strange behaviour, that some (including me) will consider a bug, while others (including the SQL Server development team) consider it to be “by design”.