The same PCR gives different results?? - PCR (Aug/10/2005 )

I run PCR in three tubes in parallel, with exactly the same reagents added in a very short time range.

However, the three tubes gives different results. One gives a PCR product band (500bp) and a heavy larger band (1.6kbp), one gives larger band (1.6kbp) only, and one gives nothing.

How can the same reaction lead to different results?
Thanks for help!

By the way, the template is a 4 kbp sized plasmid.