This is a bug that has been reported several times before, and the Slash coders don't seem to care. Basically, for those who don't know, you don't get a karma bonus for being modded up as funny (which is a bit iffy in itself -- the reasoning behind it being "don't tell jokes all the time"), but if you're modded up 5, Funny and someone mods you down as being a troll or off-topic or whatever, you still lose karma. Even if your post was a funny joke, and some asshole with a different sense of humor thinks it was a troll, your karma is on the line to said asshole. You can't GET karma for being funny, but you can LOSE it if someone thinks you're not funny. You can't deny that that's stupid, and indeed, it has made people look towards other news mediums than Slashdot.
And I may soon look with them, if the Slashdot staff continues to be this arrogant and as much of assholes as they are to people.
There was also a rumor of a post that, if modded up, the Slashdot staff would add you to an obscure variable in the Slash code which would ensure that you never get to moderate (or even meta-moderate) ever again.
And the Slashdot editors complain about censorship... HA!!