Warning (2): fopen(cakephp/app/tmp/cache/persistent/cake_core_dir_map) [function.fopen]: failed to open stream: Permission denied [CORE/cake/libs/file.php, line 146]

From time to time I get the following cakephp error

Warning (2): fopen(cakephp/app/tmp/cache/persistent/cake_core_dir_map)
[function.fopen]: failed to open stream: Permission denied [CORE/cake/
libs/file.php, line 146]

What could possibly be the reason?

I'm using the latest cakephp

Mal
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