Since 1954, forty-four qualified pitchers have allowed opponents to slug .500 over a season. Twenty have cracked the .520 plateau:
The abbreviated 1994 season and Jose Lima are both well-represented. Pitchers just didn't have as much time to a) drop their slugging percentage to something more respectable or b) get replaced before reaching the usual 162 innings to qualify. The other seven seasons all took place this decade, which makes sense as slugging percentages league-wide have reached new heights. Here's the .500+ opponent slugging allowed list prior to 1994:
Yep, that's it.
A high slugging percentage allowed is fueling Jeff Suppan's run at another distinction. He likely will not reach the required 162 innings, but if the season ended today he would be only the ninth qualifying pitcher to allow an OPS of .900 or higher. The current list of eight is the highest slugging allowed list jumbled a bit:
Suppan has allowed opponents to put up a .900 OPS this year. Cahill, if you're wondering, is second with an .856 OPS allowed.