Washington flipped to Republican in 1994. Hard.
I don't completely disagree, but it's important to remember that a combination of an open seat + several freshmen Democrats might make it look a bit more "hard" than it really was. Cantwell (her seat was won by a Republican in 1990), Krielder (his district was new in 1992), and Inslee (his seat was won by a Republican in 1990) were all freshmen. Swift retired.
Additionally, Eastern Washington moved to the Republican Party for good during the 1990s, and the fact that the Democrats had both seats in the East after 1992 was impressive, but also kind of an anomaly. Tom Foley losing was certainly upset for obvious reasons (and Unsoeld to a lesser extent, though I don't fully understand SW WA's bizarre voting patterns), though I really don't know how Jay Inslee ever got elected in the 4th district (the district voted 71% R in 1990, 51% D in 1992, and then 53% R in 1994, and now is reliably >60% R). Are there any Democratic districts that I think will trend Republican permanently like Eastern Washington did? Not really--the seat here Republicans will probably gain (the 3rd) already has a slight Republican lean and is open. I will be very surprised if such a seat doesn't fall in a wave election.
Of course we're talking about a Senate seat here, and of course the Senate seat up in 1994 was an incumbent Republican so it's a more difficult comparison, but I suspect your "hard" comment was inspired by the House seats, hence my reply.
