EMB145 FO FAILS TO MAKE CROSSING RESTRICTION DUE TO INCORRECT FMC ENTRY AND FAILURE TO HAVE NFP CHECK IT.

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ATC ISSUES INSTRUCTIONS TO CROSS 45E OF OXI AT FL250. AS PF; I INPUT THE INFO INTO THE FMS; AND THE CAPT XCHKED AND AGREED WITH MY INPUT. A FEW MOMENTS LATER I ASKED THE CAPT IF THE CTLR HAD ALSO GIVEN US THE INSTRUCTION TO CROSS AT 250 KTS; AND HE CONFIRMED THAT HE DID NOT. ORIGINALLY I HAD ADDED A 5 MI BUFFER (XING 50E OF OXI AT FL250) TO SLOW TO 250; BUT SINCE THAT WAS INCORRECT; I RE-INPUT THE INFO INTO THE FMS. I MADE AN ERROR THE SECOND TIME OF PUTTING A XING AT OXI OF FL250. I DO NOT BELIEVE THE CAPT (PNF) WAS AWARE THAT I HAD RE-INPUT THE INFO INTO THE FMS. ATC ASKED US WHAT OUR ORIGINAL INSTRUCTION WAS; AND AT THAT TIME WE REALIZED WE WEREN'T GOING TO BE ABLE TO MAKE THE ASSIGNED XING. ATC ISSUED AN EXPEDITED DSCNT TO FL250. THE CAPT WHO HAD ORIGINALLY XCHKED MY INFO WAS NOT AWARE THAT I HAD RE-INPUT THE INFO; AND THEREFORE WAS NOT ABLE TO CATCH MY MISTAKE. IMPROVED COM BTWN PLTS DURING FMS INPUT. I BELIEVE IT WAS MY COMPLACENCY THAT CAUSED ME NOT TO ALERT THE CAPT TO MY CHANGED INPUT.