I've built two more complicated kits, so it's hard to believe I'm mucking this one up, but I am.
I already messed up getting the batteries mounting underneath instead of on top. "Just flip it over!" Yeah, except I used a sharpie to label the terminal strips on the chassis. [doh!]
Anyway, the first soldering I've done I mixed up U and L, upper and lower parts of the terminal strip. Dunno why, just did. This won't really make a difference, though, right?