Is a ring in the bottleneck necessarily a sign of infection?

I brewed an oatmeal stout 6+ months ago but it tasted bad so I put it aside to see if it would taste better down the line. I went recently to grab a bottle and noticed a ring around the top of the beer line of the bottleneck in virtually every bottle. I am pretty good with sanitizing but I know that infections often show themselves like this.

For what it's worth, I made the n00b mistake of following the 1-2-3 method exactly, so when I racked the beer actually wasn't done fermenting (but did finish - I even had yeast cakes and krausen in my secondary for a while). Would anything that resulted from this be the cause for the formation of sediment in the bottleneck?

I plan on trying the beer regardless but I am hoping to understand why this may have happened, if that's possible. Thanks to everyone and I have included a picture below.

I think i've seen something similar to that in my bottles before. Looks like yeast clinging to the bottle. Give it a couple of inversions and put it in the fridge for a couple of days.

I brewed a double chocolate stout a while back and every bottle developed not a ring but a layer of scum at the top of the beer (I suspect it has to do with the fat in the chocolate I used). I inverted every bottle to break up the layer hoping it would settle to the bottom of the bottle. It didn't. The broken pieces are just floating around in the neck of the bottles. I only primaried the beer for three weeks before bottling. If I ever do this again I may change the ingredients or try a few things I don't usually do (rack to secondary, extended time before bottling, cold crashing).

I was rather disappointed and thought my beer was ruined. But I cracked one (it didn't gush, good sign) and poured in through a strainer into a glass. While it isn't exactly what I was hoping for (maybe it will improve with age) it doesn't have any bad flavours (that I can detect anyway) and it hasn't made me sick.

My suggestion is to try a bottle like you said you were going to. If it doesn't taste bad you should be good to go.

An infection in the fermenter can follow thw beer into bottles like that.
But it can also be yeast if the beer wasn't done &/or cleared well by bottling time. I've bottled when a lil misty before,& got the yeast doing what you showed. But it also sort of rained down the sides of the bottles (inside) like fine dust. Just gave'em a lil more fridge time to settle out after swirling them a bit to get it all to settle to the bottom. They tasted fine to me.

Hard to say. I've had rings, and it was definately infection (it was sour beer, so it was purposefully full of bacteria). The head space in the bottles is full of O2 which encourages the bacteria to form a little pellicle in there.

It also seems possible that some material ended up in your bottles and it's floating now, but I have not had that happen.

If you end up with gushers or a sour or vinegary taste to your stout then you'll know.

It is possible that it is an infection, but if drank fast may not be an issue. At competitions I've scored several beers that looks like this, it can go either way depending on the age and base style how apparent it is.

Did you use DME for priming? I've heard that a small krausen ring is not unheard of when using DME as a primer.