On my first visit to DC in 1990, I spent much of a weekend day riding all over the Metro system, which of course was quite a bit smaller than it is now. Included in my jaunts around that day was spending a couple hours railfanning the NEC from the Metro platform at Landover - I still remember the E60s at speed!
Even with the old paper and magnetic stripe farecards in use at the time, the fare system apparently not only knew your origin point and destination point for the calculation of the distance-based fare, but also had an idea of how reasonably long it should take you to get between the two. Since I had been within the fare-paid portion of the system for many hours at the conclusion of my day but was exiting within a few stops of the station I had boarded, the faregate decided there must be something amiss and would not open to let me out!
After some time and a lot of trying to reinsert my farecard in various gates to try to exit, a station attendant was summoned and immediately started grilling me about how having and trying to use multiple farecards to try to beat the fare was a crime, etc. I explained to him that I was a first time visitor to DC and a railfan and I had been riding around on the system all day without going through any faregates in the interim, and yes, I only had one farecard in use. He hemmed and hawed and took my farecard into his booth and checked something on his computer and then came back out and ran my farecard through the gate again. Lo and behold it opened! He let me exit, handed my farecard back to me, and I thanked him profusely and made my way back to the hotel. I only noticed later that evening that the value of the farecard had been reduced by two weekday peak period maximum distance fares!
My subsequent visits to DC have been few and far between but I remember that one well. I don't know if the new fare system works the same way as it used to, but since that experience I've pretty much kept my lingering to a minimum on any system that requires tap-in/tap-out.