There is insufficient memory available in the buffer pool

Hi all

I noticed the following error on one of my SQL Servers:

"SQLServer Error: 802, There is insufficient memory available in the buffer pool. [SQLSTATE 42000] (ConnExecuteCachableOp)"

What steps can I take to troubleshoot this error?

According to TechNet, the message explanation is:

"This is caused when the buffer pool is full and the buffer pool can not grow any larger."

I don't understand how something like this can happen, isn't that why SQL has a lazy writer - To free up pages in the buffer pool.?

I am running SQL 2008 Enterprise on Windows 2008 Enterprise.

Server has 6GB Memory and 4GB has been allocated to SQL Server by the use of the max server memory (MB) configuration option.

Thanks