What happens if your opponent attacks you, commits a blitz/shock marker and you then play an ambush(!) action card? Does the marker get aborted and returned to the player or does it go back to the supply?
Normally that's decided by the attacker's die roll but in this case I'm unsure about the process.
This cannot happen, as the Ambush card is played BEFORE the commitment of the Blitz/Shock marker.
The rules say:
AMBUSH: Play during the Opponent’s Combat phase before a regular battle (not a Counterblow or Counterattack): you become the attacker in that battle, you also get a positive CRT column shift equal to your own defensive shift terrain bonus (this includes Boxes and Mines), with a minimum of 1 Column Shift
The card should be played right after the attacker has commited its forces, before the Shock/Blitz or any Combat cards are played.