OOP: What's up with the Two-Way Mirror?!

[owlery2003]

No: HPFGUIDX 66106

What do you think about Harry's "forgetting" about the two-way mirror 
Sirius had given him? He only thinks to use it to communicate with 
Sirius in the great beyond? He's likely to go over the edge when 
he "remembers" he had this gizmo all along - why did he need to go 
through the extreme of using Umbridge's fire? I also think there's 
something to come regarding the broken mirror. Why is it broken in 
his trunk? JK makes a point of Harry covering up the pieces . . .