You can buy thick water (designed for people who have trouble swallowing) at pharmacies and every so often I’ll mess with the new guy at work by filling a water bottle with it and taking a swig first thing in the morning after a weekend before saying something like “Damn don’t you hate when you forget your water and it goes bad?” and pouring out a thick stream

It might attract dust and other particles that slightly change the taste though

The longer you let water sit out the more acidic and “stale” tasting it becomes due to build up of carbonic acid. Others have mentioned bacteria growth and particle accumulation too, yucky.