I ate half of a pre-packed chicken sandwich before realizing it was still partially frozen. Is there anything to worry about, and can I eat the rest?
Aaronut's answer is fine, I'm just coming at it from a slightly different assumption.
Sandwiches like these are common in gas stations and convenience stores in the US. You can buy them in multi-packs at grocery stores or places like Costco.
In multi-packs they are sold frozen, in convenience stores they have often been defrosted.
They're perfectly safe still frozen as they were fully cooked before being quickly frozen and presumably stayed frozen until they were defrosted in the refrigerator before sale (if they're purchased thawed).
The only way these would pose a food safety risk is if they were thawed and left in the "danger zone" for longer than 2 hours, either somewhere along the distribution chain (highly unlikely), or after purchase. That's being very conservative, BTW.
Freezing extends the safe storage lifetime of any food. I would have to assume that a frozen sandwich may have been frozen for a long time and therefore probably isn't the best quality, but it isn't any less safe than a refrigerated, "fresh" sandwich - in fact it's probably safer, although the operative word is "probably" - that's not necessarily true unless you're sure that it's been frozen the whole time.
The confusion here is probably coming from the fact that if you've tried to cook raw chicken from frozen, and you find that the center is still cold or frozen, then for sure it means that it's undercooked and not safe (yet) to consume. But sandwiches aren't made from raw chicken; the chicken will have been cooked beforehand, and then frozen, either separately or in the sandwich. You're dealing with freezing after cooking here and that's perfectly fine.