I generally never let lies slide because exploiting them is a key element of investigations. The best thing to do is to let a suspect weave as fanciful a tale as he or she can. The more complex the better. Sometimes I don't even confront them with the lies and just include them in the report. That way the Public Defender has to deal with them in court, which pretty much ensures that there will be a guilty plea instead of a trial.
The only lies I let slide involve minor traffic offenses. I don't like to mess around with taxpayers too much so if it's a fairly innocent lie, I'll let it slide at the scene, but I will still include it in my notes. I'll bring it up in court if it goes that far.
I accept procedural lies all the time. Like, "I'm sorry, hasn't that been mailed to you? I'll check. It's on my secretary's desk." I don't challenge those. They don't mean anything. They let people save face.
You could lie to me pretty easily. The thing is, I don't care about Hemingway's "bullshit detector." While I assume everyone is honest, I also kind of assume, like Mark Twain, that everyone is lying all the time. I guess I have a bit of the post-modernist/deconstructionist attitude that it's all made up. We make up everything. On the other hand, I do think there's such a thing as authenticity. I know I feel best when I feel like I'm being authentic. I feel this when I'm comfortable and not concerned about what others think of what I'm saying. Alcohol is a good fixer for this. But so is teaching, because I don't care what my students think of me.
Most of the time, if someone seems like a tourist, or a run-of-the-mill, harried business traveler, I give them the benefit of the doubt. Maybe they didn't know they couldn't bring in certain kinds of fruit. You confiscate the merchandise, but you don't really give them a hard time.