I got caught at work with a snow storm brewing, and went out in it, only to find myself at the half way point suffering from wetness, not just from the snow but from the sweat produced to ride in the stuff.
The recent warm up and the forcast for the day lulled me in false security that I didn't need to carry that stuff with me. I didn't have my boots on so my shoes were soaked, and no dry socks, cause I had cleaned out my cold weather emergency stuff. I had kept a hat and extra gloves, and put them to use, I also had my sweater, I'd brought it just in case. But the other stuff I really needed, which really doesn't take up a lot of space wasn't not available.
How long do you keep this stuff in your panniers? I suppose dry sox, shirt and shorts are always in order, but they also take up room and add weight to the commute. I suppose I have to weigh the options.

It's a day to day thing for me.

This time of year is very difficult - this morning I wore winter stuff, in the +3 C, 37 F rain and got soaked. I'm just leaving to go home, it's sunny and 18 C, 65 F !!! I brought shorts to wear home, forgot the lightweight socks. I'll either wear the damp woolies from this morning or go sockless.
I had a spare pair of rain pants with me this morning, but by the time I realized the ones I had on weren't working very well, I was half way there and soaked already.

It is difficult. In winter I always try to carry a complete dry set of riding clothes with me, except for the gore / drymaxx / whatever shell. I never carry spare boots. I tend to overdress in that section anyways, so my feet stay warm and dry regardless of weather. I do not use clipless pedals on my winter bike, making it easier for me to choose a right pair of shoes to begin with.

It is so much easier in the summer...

--J

Always carry lightweight rain-pants, a spare mid-layer(wooly jumper) and hat, and some plastic bags.
If its cold and wet, then plastic bags between your socks and shoes make a huge difference.