Here is a quick one!
Depending on your server's default PHP settings, it can happen that you will get an error of the type:
Fatal error: Allowed memory size of 1048576 bytes exhausted (tried to allocate 24576 bytes)...
The solution happens to be as easy as you could possibly wish.
In your index.php file, you can add the following line just after the PHP starting tag:
concrete5 requires at least 64MB of memory to run properly.
Alternatively, you can add the following line to your .htaccess:
php_value memory_limit 128M
Admin's note: Not all hosting companies allow memory limits to be set by users. If you're having trouble increasing your PHP memory limit, contact the company you pay for web hosting.