This is an interesting bug that seems to affect some custom templates that have not been fully adapted to 4.1x. This causes some categories with a set number of products to throw the following error:

Invalid method Mage_Catalog_Block_Product_List_Toolbar::isLastPage(Array
(
)
)

Trace:
#0 [internal function]: Varien_Object->__call(\'isLastPage\', Array)


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