In our lab we suddenly started getting amplification for a particular gene in all templates. I mean in any palsmid templates/Milli Q water/Tap water/ buffer. The primer used is gene specific for a mammalin protein. Any comments? What could be the reason? Commercial DNAse free water(!!!) when used as template gave a clean negative result.. That means reagents are clean.. then what is the reason for this false positives?

When you say that using the commercial water you got no band - were you using the same tubes of primers that you had been using before.

If the band you were getting was the same size as the one you would expect from a positive control using these gene specific primers then it would suggest that at least one of your primers is contaminated with the plasmid DNA that you've been using as a positive control.