“She luffed straight into me!”
The situation
Two boats sail close-hauled side by side, overlapped on the same tack. Green is to leeward, Blue close alongside to windward. Green luffs; Blue can't respond in time and the hulls touch. Blue protests, furious that Green turned into her.
Green changed course and caused the contact — the boat that turned into the other and hit her must be at fault.
The question
Does Blue's protest stand, since Green is the one who turned?
The ruling
No — Blue is penalized. As the windward boat she must keep clear of Green the whole time (rule 11); the leeward boat is allowed to luff. The only thing that could have saved Blue is rule 16.1: a right-of-way boat that changes course must give the other room to keep clear. If Green luffed smoothly and gave Blue a fair chance to respond, Green broke nothing and Blue's failure to keep clear is the foul. (Only a sudden, no-room slam by Green would flip it back.)
Causing the contact isn't the same as being at fault — windward must keep clear, and a fair luff is legal.
Opens the situation on the boat-length grid — scrub it and see exactly how the boats meet. Free, no account needed.
Rules cited
See the underlying rule: Rule 11 — On the Same Tack, Overlapped.