bad shave - was it the soap or the blade?

I'm with Kav here, too. Like you, I typically face issues whenever I go for new stuff. But I have learnt not to enter more than one unknown to the mix at a time. You might have not hit the sweet spot in terms of water to soap ratio, the blade may have been a dud (or it's just not the blade for you). Sometimes your skin just needs a break, too, for whichever reason; lack of sleep, environmental stress, who knows..

Ironically, I own Windsor, as well as Arlington and Gillette 7 O’Clock Black as well. And , I have shaved with them together several times. I have never had a bad blade but it can happen. DR Harris is an excellent soap but I find it does take more effort to make a great lather. I used to face lather with these soaps but have changed and now I bowl lather. I use more water (distilled) then with other soaps and I work it longer and get a bowl full of great lather. If I had to guess it may have been a bad blade. But we all have a bad day every once and a while.

A bad blade may occasionally slip through, it doesn't take much with the thinness of a DE. Harris pucks IMO are among the very best of the "mass produced" hard soaps.

A solid 9 for the soap too, but be careful with the water/soap ratio.

I usually suspect myself when I have a bad shave. Given that you're an experienced wetshaver, your technique s probably fine, but that doesn't necessarily extend to new hardware and software.

As people have stated here: when you change to something new, there's a risk that your technique will require refining to account for the characteristics of that product. It's not always necessary, but occasionally it is. In my case, since I use straights all the time, I usually find that my first shave with a safety razor after weeks of using a straight can sometimes be sub-par. I try to be conscious of the change in technique required, but muscle memory is hard to overcome sometimes. It usually only takes a shave or two to adjust back to a product I've used before, but if I try a new product it can take longer to figure out.

However, some products just don't work for some people.

So, maybe it's the soap, the blade, both, or neither.

Could be the blade, could be the combo with the razor, could be the technique that day but I'm only speaking from my own experience, sometimes I get sloppy with my technique when I'm tired and still proceeded with my shave.

It happens once in a while, even with my favorite soaps.

For what it's worth, DR Harris is alright, but it's not my best soap.