What does the error look like?

When it happens, you’ll see yellow & black stripes indicating the area of
overflow in the app UI in addition to the error message in the debug console:

How might you run into this error?

The error often occurs when a Column or Row has a child widget that is not
constrained in its size. For example, the code snippet below demonstrates a
common scenario:

In the above example, the Column tries to be wider than the space the Row
(its parent) can allocate to it, causing an overflow error.  Why does the
Column try to do that? To understand this layout behavior, you need to know
how Flutter framework performs layout:

“To perform layout, Flutter walks the render tree in a depth-first traversal
and passes down size constraints from parent to child… Children respond by
passing up a size to their parent object within the constraints the parent
established.”  – Flutter architectural overview