I find these two words being mixed up quite commonly, and it’s one of those ones that … I won’t say it annoys me, because I try to remain calm and focused on the sense of the writing in the face of errors, but it sometimes makes me a bit tense.
The incorrect usage is always in one direction of the confusion. I’ll show you what I mean …
A phase is a distinct period of time or stage (“we are doing the building work in three phases: foundations, walls and roof, with gaps to raise money in between”) and it has some complicated scientific meanings which are related to this idea of separateness and which we probably don’t need to go into here.* The verb to phase (in/out) means to carry out a process gradually (“We are phasing in the new hires so everybody doesn’t arrive at once”) and is used in those scientific contexts I talk about below.
What phased does not mean is confused or discombobulated.
To faze is to confuse, disturb or discombobulate – so the past tense is fazed. “I was fazed by the information he was bombarding me with and had to take a break”.
Faze – confuse. Phase – time period or other separate thing.
“I was not fazed when the phases of the traffic lights were altered, because I had read the notices and knew it was about to happen.”
*Oh, alright then, if you insist: in physics, it’s the relationship in time between the cycles of a system and a fixed point in time; in chemistry it’s a distinct form of matter that is separate from other forms in terms of its surface; and in zoology, it’s the variations in an animal’s colouring depending on the seasons or genetics.