If you use such function like GROUP_CONCAT you should be aware of default result size is 1024 bytes. It could lead possible problems.
For example, check the SQL query below:
set @user_ids = (SELECT GROUP_CONCAT(user_id ORDER BY user_id SEPARATOR ',') FROM user WHERE enabled = 1);
If you expect result to be list of IDs separated by comma and you have data result bigger than 1024 characters it will be truncated to default length. If you use such result in another SQL queries such as:
SELECT some_stuff FROM user_stuff WHERE user_id IN (@user_ids);
It could lead to possible logical problems when some IDs will be cropped and wrong data will be used.
To solve such problem you can change session variable using SQL query:
SET SESSION group_concat_max_len = 1000000;
Where 1000000 is maximum possible GROUP_CONCAT result size.
Also, you can set this variable globally (it is preffered way if you work with big data in most cases. Then you should add such configuration to your /etc/my.cnf
group_concat_max_len = 1000000;
and restart MySQL server to apply new default configuration.