Can't place a contribution widget on a Contribution Page

When you attempt to place a contribution widget on its own contribution page in the "Inroductory Message" section, you get the error: Illegal characters in input (potential scripting attack)

I confirmed this is a regression by replicating the problem on the Drupal demo site (4.7) and confirming it didn't exist ion the Joomla site (4.6). I'm pretty sure it didn't happen earlier in the 4.7 cycle either.