I think I didn't quite get the bottom bolts quite tight enough on the right side of my salvage bike. After sitting for months, not licensed, there is a wee wet spot on the garage floor where it sits.
i probably need to pull the axle and put the allen wrench up in there and give the handle a whack with a hammer and tighten it up to quit the seep.
Like you said the job can go smoothly but there is always room for error.
Hopefully you did it up correctly and have no leaks.