Safe basically is sodium thiosulphate.
As to overdosing it, I gotta ask what you mean by "overdosing." If you mean "put in so much that it killed my fish," yea, it's hard to do that. It takes something like 300 mg/L to start to kill fish, although it'll deplete oxygen before that.
However, sodium thiosulphate isn't a good thing to add to aquariums anyhow. It's an irritant, and it can produce some interesting chemical reactions in the aquarium. It's a carcinogen, and it tends to bioaccumulate (this is why it is not to be used on food fish). Overall, it's a pretty harsh chemical.
Its byproducts in the reactions with chlorine aren't really that bad, but you really don't want to overdose. Keep in mind that the proscribed dosage of most dechlorinators is WAY overkill anyhow -- especially in the winter. So, if you're dosing a portion of a portion of a teaspoon into the tank, you're probably way overdosing it.
Sure, you might not be noticing any problems after overdosing it, just like you can walk around with a bar of uranium stuck in the back of your shirt every Sunday for like 23-seasons without feeling sick, but eventually it's going to catch up to you. I wouldn't be surprised if you get more tumours, lower fecundity, etc with Safe than you would with a properly measured amount.
I've been pretty critical of heavy overdoses of sodium thiosulphate based dechlorinators in the past, and am again here; I want to emphasize that it's probably the best and safest method to dechlorinate water rapidly IF YOU DO NOT OVERDOSE. You can, of course, also use carbon filtration, evapouration, bubbling, etc.
