Weird filter effects

[Herb Chong]

i was shooting yesterday in the Catskills and needed an in-between ND filter 
of 4 stops because i only had my 6-stop B+W filter with me and not my 3-stop 
one. since i carry a Hi-Tech 85mm square ND filter with me in my Cokin P 
grad ND filter bag, i placed it on the filter holder in front of the 
circular polarizer and proceeded with my shooting. chimping on the LCD 
showed something odd, but i wasn't sure of the magnitude. everything looked 
OK in the viewfinder. getting back home and looking on the monitor showed 
really radical effects. anyway, here is a link to a small gallery showing a 
pair of results with a combination of Cokin circular polarizer + Hi-Tech 
4-stop ND versus Hoya Super HMC Pro 1 circular polarizer and B+W 6-stop ND 
filter. the "normal" color ones were taken with the Hoya/B+W combination 
while the "odd" ones were taken with the Cokin/Hi-Tech combination.

http://users.bestweb.net/~hchong/Temp/
what gets me is that looking through the viewfinder and just holding the 
filters up to look directly through them don't show any difference between 
the combinations.

Herb....