mysql out of available memory
For every new MySQL connection from a Client, some amount of memory will be used. So once the number of connections (which uses RAM) exceeds, you will be getting this error.

So check your application whether the connections are properly closed after using.

Also check what is the max_used_connections status variable and check whether that many connections are really getting connected from application. If application is not using that many number of connections, then it is definitely connection leak.