The loop is everywhere. And everywhere is the loop. Honour it. Love it. Fear it.
There are lots of ways you could be injecting yourself into the loop. Maybe you’re hooking into
the_content filter, or using
loop_start, the_post or
Anyway, you probably feel pretty cool. Just step in, run a quick check to see if it’s the loop you want, then do your business.
Trouble is, that loop is everywhere. And I mean everywhere. It will get used in pages and posts, sidebars, whatever random content the theme developer decided to plug into this or that page.
If you’re going to step into the loop, do everything you can to make sure you’re only modifying the loop you want to modify.
WordPress offers is_main_query() for you to weed out loops outside of the main template loop (the one that renders a page, post or custom post type). It also provides in_the_loop(), which is useful if you’re hooking into
the_content, which is very likely to get called outside of the loop at some point.