Query Limits broken in 2.5.15
From Joomla! Documentation
The "J2.5" namespace is a namespace scheduled to be archived. This page contains information for a Joomla! version which is no longer supported. It exists only as a historical reference, it will not be improved and its content may be incomplete and/or contain broken links.
Modules that involve selecting a limit number of articles in Joomla 2.5.15 ignore their limit.
What is the cause
This is because a sanity check was backported into J2.5 from J3.x that didn't function correctly.
How to fix
Please update to 2.5.16