United States v. Jorge Guerrero, 921 F.3d 895

Guerrero was a passenger in a car stopped by police after officers testified the driver failed to signal before a left turn; Guerrero moved to suppress the firearm and ammunition found during the stop.
In the district court Guerrero argued only that the officers lacked reasonable suspicion because the driver had signaled before turning; the district court conducted an evidentiary hearing, credited the officers, and denied suppression.
Guerrero pleaded guilty conditionally to being a felon in possession of a firearm, preserving the right to appeal the suppression ruling.
On appeal Guerrero advanced a new suppression theory: under California law a driver must signal only if another vehicle may be affected, and the government did not prove another vehicle could have been affected.
Guerrero did not raise that California-law theory in the district court; the Ninth Circuit addressed which standard governs appellate review of new, untimely Rule 12(b)(3) theories after the 2014 Rule 12 amendments.

Issue	Plaintiff's Argument	Defendant's Argument	Held
Standard of review for a suppression theory raised for first time on appeal	Govt: district-court factual findings should stand; appellate review constrained by defendant's failure to preserve issue	Guerrero: plain-error review under Rule 52(b) should apply to untimely Rule 12(b)(3) theories after the 2014 amendments	The Ninth Circuit applies Rule 12(c)(3)’s good-cause standard; Guerrero failed to show good cause
Whether the stop lacked lawful basis under California signaling rule	Govt: officers credibly testified driver failed to signal and that provided reasonable suspicion for the stop	Guerrero: even if driver failed to signal, Cal. Veh. Code §22107 requires proof another vehicle may have been affected; government introduced insufficient evidence	Court did not reach this new argument on the merits because it was not raised below and Guerrero did not show good cause to excuse the late theory

United States v. Caseres, 533 F.3d 1064 (9th Cir.) (evidence required that movement could affect another vehicle under state signaling law)
United States v. Keesee, 358 F.3d 1217 (9th Cir.) (pre-2014 good-cause rule: new suppression theories not raised below require good cause to be raised on appeal)
Puckett v. United States, 556 U.S. 129 (Supreme Court) (plain-error standard is default for unpreserved appellate issues)
United States v. Soto, 794 F.3d 635 (6th Cir.) (post-2014: plain-error review applied by some circuits)
United States v. Vance, 893 F.3d 763 (10th Cir.) (post-2014: applying Rule 12(c)(3) good-cause standard)