Worst SV% of a goalie in a season with at least 42 starts in the 21st century:
Jose Theodore, .882 and a GAA 3.41 for Montreal in 2005-06.
Worst GAA:
Olie Kolzig, Washington Capitals 2005-06, 3.53 in 59 GP. (SV% .896)
Neither was generally speaking a terrible goalie.
I think
John Grahame, TBL 2005-06 is a strong contender for the worst. Defending Stanley Cup champion team goes from 103 points (with Khabibulin) to 92 points and a first round exit with Grahame.
Grahame had a SV% of .889 in 57 regular season games, which was 4th worst this century, and .847 in 4 playoff games. His GAA of 4.76 in the playoffs was the worst in a decade (for goalies with at least 4 games in a playoff season) and easily the worst this century. That was his only season with 30+ starts.
Btw, never realized it back then, but 2005-06 saw some terrible numbers for starting goalies
Four of the six worst SV% in this century in that one year (Grahame, Theodore, Khabibulin for Chicago and Nabokov for San Jose).
Or to put it another way:
Number of goalies with at least 42 GP and a SV% under .900 per season:
2003-04: 2
2005-06: 11
2006-07: 6
Total between 2007-2017: 14
I guess the lockout had it's biggest effect on goalies. Increases my respect for guys like Hasek, Kipper and Lundqvist who had great years.