Spoofing is placing non-bona-fide orders with intent to cancel before execution, to create a false impression of supply or demand. It became explicitly illegal under the 2010 Dodd-Frank Act, and the 2015 United States v. Coscia case produced the first federal spoofing conviction.

Detection heuristics a tape reader can apply in real time:

Layering: multiple large orders stacked 2–5 ticks from the touch, appearing and vanishing in under 500ms.
Cancel-to-fill ratio: a participant with >95% cancels and near-zero fills at a level is likely non-bona-fide.
Asymmetric pull: size on the bid disappears the instant price approaches, then reappears one tick lower.

Behavior	Legitimate	Spoof-like
Order lifetime	Seconds to minutes	Milliseconds
Fill rate	10–40%	<1%
Reaction to price	Holds or adjusts	Pulls pre-touch

Caution: HFT market makers legitimately cancel constantly. Context — size relative to book, timing relative to news, and repetition — separates the two.