Magnetic Stirrer

Stirring will produce heat, as mechanical energy will change into heat energy.
After vigorous stirring the solution will heat up a little bit, which will make measuring of the enthalpy change of neutralization of acid and alkali incorrect by introduction of heat into the system. As the heat introduced into the system is limited by only slow stirring, the error induced will be minimized. It is not a good idea to add the mechanical work applied to the system by magnetic stirrer into account as change to heat energy is not the only form of change of the mechanical energy, it will just make the error be larger.

Affect? Not affect?
It may affect or may not affect due to the fact that it depends on you result want to get.
You want it to affect, then it is affect; You want it not to affect, then it is not affect.