While a written contract cannot be altered by parol, yet if its execution was the result of fraud, accident or mistake, such fact may be pleaded and proved by parol in avoidance thereof.

.Held, that, to a suit on the bond such facts might be pleaded and proved by parol, and would constitute a good defence. 54 Ga., 289; 52 Ib., 149; 56 Ib., 31, 32; 59 Ib., 850.