I just took Improved Critical feat for my Fighter and noticed that critical hits happen only when I roll a 20. Is this a well-known bug? I'm using rapiers, so I should get critical even if I roll 15, right?

Another question about rapier wielding: when I dual-wield two rapiers, I get +2/+2 to my attacks because, according to the game, the off-hand weapon is light. The problem is: it's not! Has anyone else encountered this bug?

(I have all game patches installed, in case you're wondering.)

Well, you need first to hit at all, and only 20 warrants a sure hit. Next, a threat-range throw (15-20 in your case) must be confirmed critical by rolling another throw against the target's AC. You hit, you have a critical. You miss, you only deal normal damage.

Also, you get no to hit bonuses whatsoever from dual-wielding. Contrary, you get penalties.

Everyone would dual-wield if it gave bonuses to hit...

And rapier is a light weapon.