Bob is deathly allergic to Coke. It doesn't matter that he drank slowly.
Why would he order it?
the ice cubes are poisoned. as bob drinks slowly, the ice cubes melts and poisons him and therefore he dies 
Good job!
Pianistimo, could you explain the peanut butter thing? Or am I just slow?
Here are some more:
1. A man, while on sentry duty, dreams of the time and place of the enemy's upcoming attack. He tells an officer, the army gets prepared, and easily wins the battle. Then the aforementioned sentry is killed. Why?
2. First think of the person who lives in disguise,
who deals in secrets and tells naught but lies.
Next, tell me what's always the last thing to mend,
The middle of middle and end of the end?
And finally give me the sound often heard
During the search for a hard-to-find word.
Now string them together and answer me this,
Which creature would you be unwilling to kiss? (if you've read Harry Potter, don't give the answer.)
3. What can run but never walks,
has a mouth but never talks,
has a head but never weeps,
has a bed but never sleeps?
4. When you say my name,
I no longer exist.
Who am I?
5. A box without hinges, key, or lid,
Yet golden treasure inside is hid
I think that's enough for now...