Take the accepted answer on this thread:
If you look at the comments, someone @'s techzilla. But who is techzilla? No one in this conversation is named techzilla. In the same comment chain, someone @'s tastro. Who is tastro?
I know users can change their names so I assume this is what is happening. These users have changed their display name or switched to their real name after their comment was posted.
Much of the time you can discern who the person was replying to by context of the conversation. In this case, Jo Smo is the only person discussing advantages so obviously Jo Smo must be tastro. techzilla is J. M. Becker after visiting his profile and seeing his LinkedIn name.
Sometimes determining the persons' name by context is a little more difficult. While it may be a semi-rare occurrence, it makes it more difficult than it should be to discern who is replying to who's comment. This problem will get increasingly worse as people decide to change their name in the future. This is more so a problem when comment chains start to become very long. (Though, perhaps there is an opinion of whether comment chains should be so long in the first place or whether people should use @user to indicate specific users.)
As I browse SO more often these days, I do run into this issue. Is there a way to avoid it and if not, is this a small enough issue to other users to ignore completely?