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What is Problems encountered during instrumentation : and explain in detail?
Problems Encountered during Instrumentation

Loss of working length due to a blockage of the canal system
Ledging (shelf formation)
Breakage of instrument
Deviation from the normal canal anatomy
Zipping (elliptication)
Stripping/lateral wall perforation
Inadequate canal preparation
Overinstrumentation (beyond apical constriction)
Over preparation (excessive removal of tooth structure)
Under preparation (insufficient canal preparation)