Will - I've noticed on a number of occasions, while riding MS 150s and with my local groups that a few people have the propensity to ride left of the center line. It's weird, but they continue to do it even if you draw it to their attention. Fortunately this is usually at low speed when they are chatting, but I do see a few who ride outside the designated bike lane for no apparent reason. We can't all be perfect!
As for pacelines, I only ride with people I feel confident with and have experience of their riding abilities and discipline. We used to have a reminder session before each ride on what to do and what not to do in a paceline, but that seems to have been neglected. I try to do this if we have a new rider and if they have never ridden in a line, we ask them to stay at the back and watch how things work.
A little while ago we had a strong rider join us who said he knew paceline riding but then proceeded to use his aerobars in the midst of the line. Others muttered about this, but no-one said anything until one evening he was in front of me in the line and I politely told him it was a no-no. No harm done and no offence taken.
Another point on pacelines, we try to limit the line to 6 riders, so we may have three groups. This minimizes the time during the rotation that the changeover rider is out in traffic.
Regarding the comment by someone about A and B groups, our experience has been the opposite. We ride at 18-20mph in the B group. There is another group which rides in the 20-22mph range, but they have had several serious accidents due to carelessness in the past couple of years. I have ridden with them, but soon realised they were an accident waiting to happen.