A little bit of paper ingestion is usually not a problem. The ink itself should be non toxic. The only worries are when large pieces or large amounts are ingested, as this has the potential to cause a GI obstruction. Paper doesn't disintegrate quickly and may not get chewed up into small pieces that could pass easily, and so can form into a gooey ball that can lead to an obstruction occurring. It's rare, but it can happen. Also best to avoid using boxes with the shiny paper coating, as these contain plastics.
Ingestion of coarse hay does help improve gut motility, so would help push gi contents through, provided there isn't a large enough mass of paper to form an obstruction.
If it was just a small piece, it's not something I would worry about.
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