Sacrifice - rules, guidelines and modern issues

 What if the animal develops a defect at the time of slaughter?

If at the time of slaughter, the animal jumps and one of its legs breaks or somehow it develops some other defect, then this defect does not affect the sacrifice.  For e.g. if the animal jumps, develops a defect right at the time of slaughter and frees itself from the person/s handling it and is again recaptured, even then the sacrifice is valid.  As mentioned in Radd Ul Muhtaar:

(Radd Ul Muhtaar, Vol. 5, Pg. No. 229)