Accidental etiquette breach

Don't sweat it. Should you have checked to see what was behind you? Probably, but it is also the paceline's job to adjust to the changes going on in front of them, so I wouldn't think twice about it.

Don't worry about it - you didn't commit any cycling sin. Sure you could have dropped to the back, or you could just swap leads. Both are fine.

And I'm jealous of your NYC Central Park ride. A couple years ago I ran that loop when I was visiting in October and it was very pretty. I bet late April it's very nice.

My worst accidental breach was in an Olympic tri, my first "real" race. I was on the bike hammering along and caught up with some woman I'd been trailing for a few miles, and passed in front of her. Just then I looked at my heart rate monitor and I was way too high, like 175 when it should have been 150. So I immediately slowed down from like 22 to 18.

From her perspective, what I had just done was cut in front of her and then slow down - and that's true, but not intentionally. Man she was pissed and called me a jerk and few other unprintables. I apoligized but really, the damage was done.

I've never made that mistake again.

I'd think if anything, the guys that came up behind you and formed the paceline were breaking the etiquette to start with.  You don't just go up behind someone and draft them, it's dangerous for you and the guy you're drafting.  Unless of course you ask or the guy you're about to draft (you in this case) acknowledges you and gives the ok.

As for after the paceline is formed and you've basically said ok to it, when you drop back I think it's fine that you pulled in behind him, it goes on sometimes.  The guy behind you obviously wasn't inches behind from the guy that came up beside you, otherwise there's no way you'd have missed him.  I'd suspect the 3rd guy in line was either still on your wheel or had dropped back to let the conversation about who will lead go on and figured one of the two of you would fill that gap after it was decided.