For iPhone LCD & Screen replacement ,if you are professional shop for customers .sometimes it's possible to meet cracked inside situation .you can see below pictures. 
the common situation for cracked LCD inside is you can always find "BLACK BAR OR AREA".

iphone cracked LCD

This situation happens the most from the logistics problem .logistic company / post office might not care enough for the parcel .so there's some external force applied during the delivery process.
( if in this case .please contact Sintron for support and get a new one )

however .there are also some cases are that new beginner uses the wrong screws an then penetrate the LCD .so got it cracked. 

if you know how to tear off the layer of back light .you can see the cracked area as picture below .