Make every drag-and-drop action cancellable
G210 asks that a pointer action started by mistake can be abandoned before it takes effect. W3C names three routes and requires one. Release outside a drop area and nothing happens. Drag the item back where it started, as a separate undoing action. Or add a step after the drop, a confirmation dialog or an undo command. Pressing Escape mid-drag is a good pattern and it is not one of W3C's three, though it serves the same purpose. This is a sufficient technique for 2.5.2 Pointer Cancellation at Level A, one of two for that rule. It covers author-built interactions only, not browser gestures such as swiping through history and not the operating system's own. W3C adds a warning that applies to every drag interface. Author-supplied path-based and multipoint gestures usually stop working when a screen reader is running, because the assistive technology consumes them first.
How we find it in an audit
Our reviewers start each drag and then try to get out of it. Release outside a target, drag back, look for an undo. We also run the interface with a screen reader active, because that is where author gestures tend to stop responding, and a drag interface with no non-drag alternative fails a separate rule as well.
How affected users experience it
Somebody with a hand tremor does not choose where the pointer lands. On a board that commits the moment a card leaves its column, a shake mid-drag files the card wherever the tremor left it, and there may be no way back. The cost is not a mis-click. It is work quietly moved somewhere nobody will look for it, and the person who did it may not know it happened.
Passes vs. fails
Passes
Cards dropped outside any column snap back, a confirmation follows any cross-column move, and a 'Move undone' control reverses the last drop.
Fails
A kanban board that saves the card the instant it leaves its column, so a tremor mid-drag files it wherever it falls, with no undo.
Other ways to satisfy this rule
3 guides on this site are filed under 2.5.2 Pointer Cancellation. W3C lists this one as sufficient for that rule on its own. Implement it correctly, in a way your readers' software actually supports, and the rule is met.
This guide is our interpretation of W3C technique G210: Ensuring that drag-and-drop actions can be cancelled. W3C publishes its techniques as guidance rather than as the standard, and says so on every one of them. The success criterion is what conformance is measured against, and a technique is one documented way to meet it.