Wage decreased without my knowledge

I'm in Washington state.

I've worked at a place for almost 8 years and worked my way to management, so I know how some of this stuff works. I was given a raise three months ago and another last week. They accidentally gave me too high of a raise three months ago, and now they decreased my pay to "make up for it". They did this without my consent or knowledge which I know is illegal. They didn't count on me doing the math. When I approached my boss he backpeddaled a lot and generally gave a bunch of BS. He claimed that it was only for one pay period, which I doubt would have been the case if I hadn't noticed. I know of similar instances where they have forgotten to enter someone's raise into the system and then changed the date on payroll status sheets so they don't have to retro-pay (or whatever you do to rectify that).

I want to know if I can collect unemployment if I quit. I called the unemployment offices and they can't give me an answer, but maybe someone has seen a similar case? If not, what other actions can I take?

[Bricks41]

If you flat out quit because of this, no you will not qualify for unemployment just based on the fact you quit. If it is a circumstance of "Resign or else we will terminate" then you can still obtain benefits.