The grapes/raisins thing is semi-recent - while renal failure in dogs fed grapes or raisins was first noticed as early as 1989, it really was not a publicized risk until 2001. So far a specific mechanism is not known - no pesticides, mycotoxins, etc. have been common to all cases, and both commercially and organically/privately grown grapes have caused problems. One or two grapes or raisins for a Boxer is not something about which to panic, but until (unless) more is known about why these have become a problem, it's probably best to avoid them for the most part.
Garlic can be problematic in large doses, but the health benefits support its use as a supplement in moderation. Garlic is from the same family as onions, and so can lead to the same problem, but onions have a much higher concentration of the toxin.