If it's new it's defective. If it's used and used to be straight, the body is failing -- possibly form a high impact on a sharp edge, like the far lip of a pothole -- and will let go pretty soon.
In any case, you'll probably feel it as a bit of a wiggle every time it comes around. These usually ride like crap, so while it might be safe, you really don't want to ride it any longer than necessary.
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