When you create Drupal code for 7 years, it’s easy to mistakenly assume that you know everything regarding such basic things as theme_preprocess_node() and drupal_get_form() functions.
And that is a wrong feeling. Today I’ve discovered that you shouldn’t mix these functions!

The bug description:
I was creating “join” form which was rendered in node.tpl.php of organic group.
When the form was submitted, validation/status messages were shown only after additional page refresh. So, after form submit page was reloading without any messages, and all the messages were appearing on the screen after additional page reload.

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