Railing post ended up right in front of my basement window - now what?

I had basially the same situation on a deck I built for my sister a few years back. Post landed right in front of a basement window and I caught it after the frame was mostly done. I went the angled rail route with the triangular gap, put a big cedar planter in that corner, and honestly it looks intentional. People ask about the planter all the time and nobody has ever guessed it was covering up a layout mistake. The angled section reads fine as long as you keep the angle shallow and use the same baluster spacing through the whole run so it doesn't look like an afterthought.

Been framing decks since before laser measures were a thing, and I can tell you the tape-and-chalk-line layout was where that post got lost. We always snapped the whole layout on the rim before cutting anything for exactly this reason. That said, your planter idea works fine. One thing to check before you commit though: make sure that basement window is an egress window. If it is, code may require clear access below it, and some inspectors get picky about anything blocking the escape path, railing or planter included. Worth a quick look at your local rules before you build the box.

We had the mirror version of this on our back deck, except the post landed where a downspout was going to run. I moved ours over about 14 inches and it honestly took an afternoon. Cut the new rim section, sistered the joint with some construction screws and joist tape, done by dinner. The thing that actually made it painless was a cheap laser level I'd picked up that summer. Set it on the existing rim, marked everything at once, no transfer errors like I got with the tape the first time. If your rim rework is just sliding a post a foot, I'd do that instead of living with a diagonal rail for the next 20 years. But if the framing fights you, the planter route is a fine plan B.

One thing with the planter corner, watch where the water goes. Ours sat right against the siding for a season and it kept that whole patch damp, ended up with some staining on the sheathing when I finally peeked behind it. If you do the planter, line it and drill drainage holes that point away from the house, or just build a bench there instead. A bench solved it for us and honestly gets way more use than plants ever did.