The slip up ocurs right before Harry Dalton (Pierce Brosnan) has the the town meeting to announce officialy that the town is in grave danger: the day before he goes to the house of Rachel Wando (Linda Hamilton), he finds that the town's water is contaminated as he samples the water in Rachel's home (he gets water from the kitchen sink) and is all muddy).He then proceeds to go to the town's water reservoir or water treatment plant (don't know which) and he truly discovers that the water is contaminated.

The very next day,he gathers everyone in a town meeting but there's a shot of him sitting on a table and on top of that table there's a glass of real clean water!

So much for the water being contaminated