Adding too much MgCl2 in your PCR reaction enhances enzymatic activity. This can promote non-specific binding of primers leading to errors in DNA replication. The agarose gel electrophoresis will show too many DNA bands. 

Too much MgCl2 also increases the chances of primer dimer formation. 

Adding the right MgCl2 concentration is essential to avoid getting any errors. The ideal concentration range of MgCl2in PCR mixtures is 1.5 to 4.5 mM. A final concentration of 1.5 mMis optimal for satisfactory yield in most applications. A few may require up to 4.5 mM MgCl2. Very rarely will you need to add more.