LUT hazards

--- Quote Start ---

I don't quite understand the concept about multiple inputs changing at the same time, because two signals will never change exactly at the same time.

--- Quote End ---

To prevent glitches on the LUT output, you must not let more than one input change at approximately the same time. There isn't a spec or a timing report saying just how far apart in time toggles on two inputs must be to guarantee no glitch on the output. This doesn't matter for a synchronous design with the logic feeding a register.

More than one input toggling at about the same time matters for something like a latch where the data and enable must not change at about the same time for a D latch and the set and reset must not change at about the same time for an SR latch. There are in effect setup and hold requirements between the latch inputs, but Quartus doesn't analyze that timing.

More than one input toggling at about the same time matters for combinational logic that must not be allowed to glitch like a clock or an asynchronous reset. There are ways to avoid glitches in these applications without having to know how close in time two LUT inputs are allowed to toggle; see http://www.alteraforum.com/forum/showthread.php?p=8506#post8506.