I started mining at xtrapool.com (8th January) then noticed that the pool wasn't confirming coins even after many times the confirmations required.  I stopped and soon after found that I was getting login errors on other pools where I had used the same username.  xtrapool is no longer around so maybe this is where it started.  Lockouts started happening on other pools like n0nplusultra.net and valid-error.com soon after I had registered with xtrapool - I have since created new accounts so that I don't use the same username - a pity, but it appears that it is a necessity.  I like the way that other pools now require you to login with your email address, but displays the username if you don't set it to anonymous.