If you use theme with theme.html (some for 2.3.0), after update to 2.3.1 you will see this error:

Fatal error: Class theme contains 1 abstract method and must therefore be declared abstract or implement the remaining methods

You can look for an updated theme version, but if there is no one, the fix is easy:

1. rename _construct() to init()

2. move  e107::lan('theme');  before class itself