Fastboot oem unlock is a command many Nexus owners know by heart. The command, which unlocks a Nexus device's bootloader, takes a special consideration in Android 5.0 Lollipop, though. The command will fail unless a certain box is checked in the device's "Developer Options" menu. This is a minor change, but one that isn't immediately obvious to the user.
On my Nexus 6 review unit, the option was already checked after I opened Developer Options, but it appears that may not always be the case, as one tipster indicated the option on the Nexus 9 had to be checked manually.
As a quick reminder, the Developer Options menu is unlocked by tapping your device's build until the on-screen countdown completes.