I've had this happen several times recently:
I'm attacking a Very Tiny planet with 20 population and 0 defense. I hit it once, reducing the population to 1. I click the "invade" button and get an error because the population is too numerous for me to attempt invasion. So I attack it a second time, again reducing the population to 1. This time it lets me invade.

It has happened to me once or twice on a larger planet (population shows as less than 10 but I get the "too much population" error when I try to invade), but I haven't invaded any of those recently to be able to say how frequently.

Mostly it's just annoying, but it's happening repeatedly so something is wonky there.

What's happening is kinda 2 glitches at once. When you attack a planet, for some reason the damage you do doesn't actually get counted (glitch #1) but the damage you think you do is knocked off the planet's population bar (glitch #2). If you attack once and close the planet, then reopen it, you'll see that no damage was actually done. Subsequent attacks seem to be processed normally, however. This has been brought to Dan's attention on numerous occasions.

Okay, thanks. I scanned through some of this forum but nothing jumped out at me as being this issue, so I thought I'd just post it. Good to know he's already been made aware of it.

You should tell that to Pony.

Mission Exotic?

But he said several times recently...