When any person shall unwittingly or by mistake, erect any fence on the land of another, and when by a line legally determined that fact shall be ascertained, such person may enter upon the premises and remove such fence at any time within three months after such line has been run as aforesaid: PROVIDED, That when the fence to be removed forms any part of a fence enclosing a field of the other party having a crop thereon, such first person shall not remove such fence until such crop might, with reasonable diligence, have been gathered and secured, although more than three months may have elapsed since such division line was run.