I do not believe there is any possibility that somebody's N-400 application would be denied or that somebody's citizenship would later on be revoked simply because the person did not report a verbal warning by a police officer on their N-400.
Ah, yes. This is the key point to me. Is this possibility real? Can the USCIS look up traffic stops, and do they?
So, the opinions in this thread boil down to two main ones:
- DO NOT report (as detention) traffic stops with only verbal warnings, questioning by police, and other such minor stuff, because you will be red flagged and your application will take longer, or be rejected.
- DO report the above, because you may be concealing something from USCIS that has a written record. This is a minority opinion shared by only a few folks (specifically, dafortycal).
My policy is that I better damn well report anything that has a written record. So what if my application gets red-flagged because I had some verbal warnings? I did not do anything wrong, in the Good Moral Character sense. It may take more time and go through more hands, but it should not get rejected -- whoever heard of an application getting rejected for traffic stops? The only concern is if you have a lot of those in conjunction with more serious offenses (this is true in my case, see
http://forums.immigration.com/showthread.php?t=292133 -- yet my IO did not care about the combination of traffic stops and an arrest).
On the other hand, if I fail to report something for which there is a written record, as dafortycal suggested, I may be committing perjury, and that perjury can be established by following a paper trail. But will anyone follow that trail? What are the odds? And if someone does, how well can you do on appeal? Will someone listen to your arguments that you left these things off your N-400 with "good intentions"?
I am not trying to "show off my honesty", I am pointing out that this question is really about accurately estimating which probability is greater:
- the probability of USCIS rejecting an application because you got very detailed on Part 10, or
- the probability of rejecting it because the USCIS looked up a written report from an officer (even for a stupid dumb thing like a traffic stop)
The bottom line is that none of us has the data to estimate these two probabilities accurately. It becomes a personal choice based almost on faith of sorts. I chose to omit verbal warnings and later tried to append them to my application at the interview. The IO did not care (then again my IO seemed very rushed at the interview). Your mileage may vary.
And why are we so afraid of supervisors looking at our applications anyway?
But I certainly would NOT endorse answering YES to "have you ever claimed to be a citizen..." question because I may have mentioned it in a conversation with friends.
It is important to remember this rule of thumb: Never volunteer information to any authority, unless you want to be screwed. For example, at your interview, the officer ask you whether you had breakfast that morning. Your answer should YES. Don't tell him how you had eggs with salmon, rice bread and cereal, plus chocolate milk. He doesn't care about what got into your stomach. 
I agree with Vorpal, you were never detained, but it is possible that an incident report exist somewhere with you "agreeing" with the officer that you were following someone becase they were smiling at you...
Beware of anyone who appear interested in your for no good reason....that includes men...