application vigilant concurrency concerns

in asp tutorials, , i observe following pattern:

application.lock

'do things concentration object

application.unlock

however, given web pages have churned instances, there an apparent concurrency problem. questions following:

what page tries close while vigilant already locked?

is there proceed detect presumably concentration vigilant locked?

is improved only work an unbarred concentration vigilant does have consequences?

what there wholly movement involving concentration object? ~is there reason lock/unlock case?