I'm going to conduct a primarily VFR flight with a destination that is forecast to be IFR at time of landing. It's also a destination that has flow control, so I file a "Z" composite flight plan, meaning VFR then IFR, so I essentially have my name on the list for the IFR arrival. The IFR planners don't like something about my plan, so they call me back and ask about it. I explain in more detail what I want to do, and then they tell me what I have to file in order to get that. It's less like what I want to do, but if that's what they want, then I'll file that.
We fly. It's night. It's pretty quiet. In flight I get periodic updates on weather at destination. The airport has an ILS, which puts my minima at 200' and 1/2 mile visibility. It's now 500' overcast and three miles, which will be a nice exercise in doing everything right ending in a comfortable margin below the clouds to transition to landing. And then it's 300' overcast and 3/4 mile. That's still something that anyone who is instrument qualified should be able to do, but leaves much less room for error. And then the next report is 500' overcast and half a mile. I recheck the weather and forecast at my alternate. Still good. My alternate is closer, but I'll try the big airport at minimum advisory visibility. I'm going slower than the big jets, so I'll have more time in that last 300' of descent to distinguish the runway lights through the mist. And if I miss I miss, another good exercise, and head to my alternate.
I'm ready to check in with the approach controller but Centre tells me that they are not permitting Cat I aircraft to attempt approaches at my destination. That means that since I don't have autoland or other special equipment and training to land in lower than half a mile visibility, they aren't even going to let me try. I feel, well for my initial notes on this flight I wrote "blue balls." I'm all set up and mentally prepared for the ILS and then told I can't play. But I understand. They are trying to get heavies on the ground there, have to increase their usual separation and they don't want a light aircraft tying up the facility and then going missed and making them have to hold the heavies out of my way.
I tell Centre I'll land at my alternate. Their ATIS notes they have a fog bank encroaching from the south. Oh, fun. They are still VFR. They have me keep my speed up because there's a fast jet behind me, but I get all the right bits sticking out at the last minute and put it down. Exit, taxi cautiously in dropping visibility. I advise the ground controller that this was a diversion, here overnight but don't know where to park. We were thinking of waiting it out here for a bit and then trying the big airport in the wee hours of the morning, when either the weather might have improved or they weren't so busy, but by the time I've parked in what the ground controller assures me is a good spot I really have to take his word for it, as it's 1/4 mile in fog and 200' here. We call it a night and take a long cab ride to the destination. Where the weather is now four miles and 800'. Argh. I was going to say you can't win with fog, but when you get the airplane on the ground, it's a win.