In this case it turned out to be simple: the test failed because it was making wrong assumptions after a code change. But I never got to know, as PHPUnit ran out of memory... This is the end of the XDebug trace:

 42.4811  96481024 -6824    -> PHPUnit_Framework_Constraint_IsEqual->fail(class F3\Fluid\Core\Parser\SyntaxTree\RootNode, string(36), ???)
42.4812  96481136  +112      -> PHPUnit_Framework_Constraint->failureDescription(class F3\Fluid\Core\Parser\SyntaxTree\RootNode, string(36), bool)
42.4812  96481216   +80        -> PHPUnit_Framework_Constraint->customFailureDescription(class F3\Fluid\Core\Parser\SyntaxTree\RootNode, string(36), bool)
42.4812  96481440  +224        -> PHPUnit_Util_Type::toString(class F3\Fluid\Core\Parser\SyntaxTree\RootNode, ???)
42.4813  96481616  +176          -> is_array(class F3\Fluid\Core\Parser\SyntaxTree\RootNode)
42.4813  96481696   +80          -> is_object(class F3\Fluid\Core\Parser\SyntaxTree\RootNode)
42.4813  96481832  +136          -> print_r(class F3\Fluid\Core\Parser\SyntaxTree\RootNode, bool)
51.5357 441897744
TRACE END   [2009-11-21 09:59:22]

Notice the print_r line? The second field is the memory used. PHPUnit trying to get a diff of that turns out is too much. Well, around 450 MByte of variable dump - I cannot blame anyone. :)