Anytime you use a float and the containing div of that float isn’t big enough to hold it, the floated element will overflow the containing div unless you clear it.

Example:
A blog page displaying 10 posts, some with floated images. Some posts are fine because they don’t have large, aligned images or have enough text to balance things out. But other posts with images aligned to the left or right and little text seem to be running into the post below them.
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