My wife's battery vented for no apparent reason at around 70k miles. Our mechanic pointed it out when we took it in to have weak braking diagnosed- apparently the acid sprayed on a relay that controls the brake booster vacuum. Being the suspicious sort, I just assumed that the mechanic had accidently shorted the battery with a wrench or something... maybe not, though. I topped off the battery with distilled water, and there have been no problems since then.