FLC OF A B737 OVERSHOT DSCNT ASSIGNED ALT DUE TO INCORRECT ALT GIVEN BY CTLR. BOTH FLC MEMBERS ACKNOWLEDGED ALT; BUT THOUGHT IT WAS AN INAPPROPRIATE ALT FOR THEIR DIRECTION OF FLT. ATC INTERVENED AND OBSERVED THEM GO BEYOND THE ALT AT WHICH ATC REALLY WAS THINKING.

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WE WERE FLYING ALONG AT FL310; EXPERIENCING LIGHT TURB; AND REQUESTED LOWER. ZDV THEN ASSIGNED US FL250. AS WE WERE DSNDING THROUGH APPROX 26000 FT; THEY ASKED US OUR ASSIGNED ALT. WE HAD READ BACK FL250 EARLIER; AND REPEATED THAT TO THEM. HE SAID THAT HE ASSIGNED US FL280. WE LEVELED OFF AT FL250 AND HE SAID TO STAY THERE. WE (BOTH PLTS) HEARD FL250 AS OUR ASSIGNED ALT; AND READ IT BACK THAT WAY. WE BELIEVE ZDV ASSIGNED US FL250 IN ERROR; BUT WE DID AS WE WERE TOLD. THERE WERE NO APPARENT TFC CONFLICTS AS A RESULT. THE ZDV CTLR WAS VERY BUSY AT THE TIME DUE TO NUMEROUS ACFT REQUESTING LOWER ALTS DUE TO TURB. WE EARLIER QUESTIONED (IN THE COCKPIT) WHY HE ASSIGNED US FL250 VICE FL280 OR FL260 (PROPER CRUISING ALT FOR OUR WBOUND COURSE); BUT NOT ON THE RADIO TO HIM. HE (ZDV); 5 MINS LATER; HAD 2 OTHER ACFT TALK TO HIM ABOUT 'WHAT FLT LEVEL' THEY WERE ASSIGNED; ETC. I BELIEVE THE CTLR WAS TASK SATURATED AND COULD NOT HANDLE THE VOLUME OF ACFT AT THAT CERTAIN TIME. THIS OCCURRED AT XX00Z; FREQ 128.12 NEAR DOVE CREEK VOR (DVC).