The smell from eating asparagus
Apparently, both are true.


  ...Individual differences exist in both odorant production and
  odor perception. The biological basis for the inability to produce the metabolite in detectable quantities is unknown, but the inability to smell the odor is associated with a single nucleotide polymorphism (rs4481887) within a 50-gene cluster of olfactory receptors. Pelchet et al. 2010




It's an open access paper (yay!), but to cut a long story short:


Pelchet et al. asked subjects to provide 'before-eating-asparagus'
and 'after-eating-asparagus' urine.
The subjects then acted as
urine-smellers for their own and others samples, and were asked to
choose between two unlabeled flasks which contained the 'before'
urine and which contained the 'asparagus' urine.
Most subjects produced 'asparagus' urine in which an odor was perceived
significantly more times than expected by chance, and
Most subjects perceived 'asparagus' urine correctly significantly more times than
expected by chance, however
~8% of subjects were non-producers, and
~6% of subjects were non-smellers.
Finally, looking at the actual amount of successes in perception: for
Caucasian subjects, one third of the variation in the amount of
correct odor perception was related to the A allele of rs4481887
(having AA or AG rather than GG). There was no connection between
this allele and production.




Pelchat, ML, C Bykowski, FF Duke, and DR Reed. 2010. Excretion and perception of a characteristic odor in urine after asparagus ingestion; A psychophysical and genetic study. Chemical Senses doi:10.1093/chemse/bjq081