err on the side of caution idiom
宁愿过于谨慎也不要冒险犯错误
to be especially careful rather than taking a risk or making a mistake
25 people have replied to the invitation, but I've erred on the side of caution and put out 30 chairs. 25人回复了邀请函，但为谨慎起见我准备了30把椅子。