B757-300
Air Traffic Control Fully Certified
Communication Breakdown
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance
Human Factors
I meant to issue FL 330 but actually issued FL300.
I relieved Sector and was briefed that Air Carrier X was stopped at FL330 and the data block indicated the same. Air Carrier Y was a ZZZ arrival at FL320 westbound and Air Carrier X was a ZZZ1 inbound northbound. Air Carrier X's altitude was and interim altitude and I noticed it showed 100 feet low and thought it was because of the interim. If we use interim altitudes; the altitude will show low/high at times but with (hard) altitudes in the data block the computer allows the 300 feet up or down without actually indicating it on the data block. I thought it was from the interim altitude until I saw the altitude drop again and then questioned the pilot. I asked him to verify level at FL330 and he replied he was given FL300 and read it back. I had descended the ZZZ inbound to FL300 and told the Air Carrier X to maintain FL330. He came back and said he was getting ready to go through FL320. The aircraft were then at least 5 miles apart so I continued his descent to FL300 per LOA with [other Center]. He said he was given FL300 and he could check the tapes. I then told the Supervisor about the situation and he called back the Controller that I had relieved. The tapes were checked and the Controller had issued FL300 to Air Carrier X and just didn't realize it. I think this was just a memory slip from the Controller I relieved but I should have said something to Air Carrier X right when I noticed the altitude.
Enroute Controller issued FL300 when thinking FL330 was issued which lead to a loss of separation.