A bug introduced in 2.5.1 would remove the parse error in the editor as soon as analysis failed. This bug has been fixed.
A whole host of bugs has been fixed in FlowSpec, mostly ones that lead to no clear error message. Much of the system has also been optimized for speed.
Separate compilation for Stratego was added in this release. It is currently still experimental. It is documented as a separate item under the Stratego documentation, including instructions on how to opt-in to it and what its limitations are.
The Stratego primitives
one sometimes lost annotations and origins of list tails when an element in the list was transformed. This bug has been fixed.