wrong cpu into mobo socket can and often still posts depending on the permutation of cpu and mobo models
also there are known i3 boards which only require bios update to work with i5. he didnt specify what board hes using, if he updated bios on an old board, etc.
using older boards often cause problems when you try to do anything intensive because certain things wont be supported by the board that the new cpu includes, most often integrated graphics are at best bugged and other features are severely limited
furthermore these old boards can, like you say break your cpu, but this isnt always the case