Problem about members receiving ban messages

My members complaining about them unable to login due to the message like "YOU HAVE BEEN BANNED FROM THIS FORUM". But when I checked, they are active, and not included in the banned list. What is causing this problem?

Is it because their IP addresses might be banned? There are two ways to ban people, by banning their user, or by banning their IP. If you banned an IP recently, especially if you banned using a wildcard like 65.128.* you may have banned them as well.

No im absolutely sure their IPs not banned too or their emails. :| Maybe it is because they are using a mobile phone with opera mini? I banned one member using opera mini but I didnt use the IP or email way of banning. I just click the link "BAN THIS MEMBER".