You can check the list of forum members who you ignored on your forum profile and you can see the ignore options on the left side under the personal message options. I am surprised that you won't be able to see that a forum member is unintentionally ingored when I can see it right away even when I am scrolling. Maybe you scroll it too fast because when it happened to me when I using mobile phone is that I will see it and then unignore the user when it happens. You don't have to worry about it since it is possible to check the ignore list as others have explained.
Most posters do neither read OP nor previous posts in a thread before posting their replies, so with this posting style, they are more easily to miss ignored users. It's not reason for all posters of their lack of realization on unwanted ignored posters in the same thread but I believe it is one of most common reasons.
Your reply describes a process of scrolling up and down in a page of thread, to read posts, and by this practical style, there is no chance of missing ignored users. If you see usernames of ignored users, and want to re check that whether they are your actual ignored users, it's very easy, click on Unignore to read their posts, check quality and decide to continue with Ignore or change it.
"Read before posting" can help to correct mistakes.
