If you have a user mailbox, and, have the forward option checked, to deliver mail to the user and forward address, activation of a user might be impossible, because the BES probably gets the activation mail from the forward address earlier then from the original mailbox.
Activation then will be stopped with the Error: wrong username or password.

I don't think I've ever used that feature to forward to another internal email address.

I also never did, but, actually someone else.

And, I never saw this as a thing to check for, when there is a problem with the activation.

I once troubleshooted (wasted rather) several hours on his BB EA only to find that the user set himself to forward all his Lotus Notes email to his personal Yahoo account. He set this via the Domino Directory in his Person document in the "Forwarding address". From then on, we made that field uneditable and this issue never happen again for newer users.

Possible scenrio: A manager (jdoe@company.com) decides to forward their email to an assistant (assistant@company.com) while they are away. Something happens that requires the Manager's BB to re-activate....BINGO!

As soon as the ETP message arrives in the Inbox the EA process starts....as soon as the very same ETP message arrives in the assistants Inbox....the BES sends a notification to the BlackBerry that initiated the EA to instruct it to stop the EA even though it was about to EA the managers BlackBerry.

Seen this many times before.

That's why there's a specific feature called delegates - it's made for that exact scenario.