  We report on an experimental study of the effect of Mn impurities in the
optimally doped LaFeAsO$_{0.89}$F$_{0.11}$ compound. The results show that a
very tiny amount of Mn, of the order of 0.1\%, is enough to destroy
superconductivity and to recover at low temperatures both the magnetic ground
state and the orthorhombic structure of the pristine LaFeAsO parent compound.
The results are discussed within a model where electron correlations enhance
the Ruderman-Kittel-Kasuya-Yosida interaction among impurities.
