Short rant this time, I just didn't get that angry this week. Perhaps next week I'll be more apoplectic.
Many arguments (such as they can be named that) invoke a worst-case scenario built upon extending an existing trend to a logical extreme. I see this all the time (I read many blogs), but it's generally subtle, and the underlying logic looks solid, so it's difficult to debunk in a short comment.
Some anonymized (and silly) examples:
Illegal immigrants will swamp our society with uneducated, unskilled, desparate people!
OK, there are legitimate, detailed and interesting debates to be had on the subject of immigration, legal and otherwise. But invoking a dark future by extending a small data sample (eg, a container truck full of suffocated Mexicans) to an entire state or country, over a timeline of decades, constitutes extrapolation beyond your data. The shortest, easiest answer to this, besides "you're a right-wing fuckwit" is to point out that immigrants come to Western countries because Western countries provide a higher quality of life. If that quality of life is somehow degraded (like, by massive influx of stupid criminals), immigration will cease as potential migrants decide to go somewhere else or stay put.
The terrorists are everywhere!
While it's probably true that would-be terrorists are present in nearly every medium-to-large city on Earth, this does not imply that they are a) common enough to matter or b) uniformly distributed. Finding a cell of individuals in some possibly-unexpected (by the dim-witted) place does not mean the guy down the street is stockpiling fertilizer for nefarious purposes.
Those people are (evil / dangerous / insane), so everybody near them is probably (evil / dangerous / insane), as well!
Sampling, folks, it's called sampling. Expressed in the simplistic language I used above, this is an obvious fallacy, not normally in need of debunking (except perhaps at architypal witch-burnings or in infantry combat - don't stand next to a man throwing shit). However, there are abundant, more subtle versions presented on the 'web. Just because Fred Phelps is obviously bad-shit goat-raping crazy, doesn't necessarily mean everyone in Kansas is crazy. Probably there are at least 12, maybe 15 non-crazy people in Kansas, possibly hiding in a tornado cellar.
OK, pretty lame rant, I know. I'd like to point out I didn't actually rant about anything particularly serious, keeping to my trivial-but-real promise of last week. I didn't rant about immigration, terrorism and global politics, or bigotry, I ranted about stupid argument tactics.