Recruiter screen turned into an actual technical question about Big O and I froze because I hadn't prepped for that round at all

Went in expecting the usual "tell me about yourself" and salary range questions, figured I'd save the real prep for the technical round. Recruiter casually asked me to walk through the time complexity of something I'd mentioned on my resume and my brain just went blank for a solid five seconds. Recovered enough to give a decent answer, but it was a good reminder that "just a screen" doesn't mean nobody's going to ask you anything real.