There is NO excuse for this!
That is irresponsibility on the part of the person who preformed the "euth" procedure! Any certified euth. tech should know better! You have to check EVERY animal for signs of a heart beat before disposing of their bodies.
The shelter i work for is a city shelter- so that means sometimes we have to make the unfortunate decision to pts animals due to aggression,illness,etc. That being said- very strict procedures must be followed- and the health dpt/etc checks in on us and other local shelter frequently to make sure all the shelters in our area are following guidelines.
The proper way to pts an animal (i'm refering specifically to dogs here, but cats too...) is to give them a sedative (ketaset, ace,etc) and then once the sedative kicks in, administer the euth. solution in the vein (in most cases, now sometimes if blood pressure is low, you use other means/etc). Once you've administered the solution- you always get the scope out and check for a heart beat- you only bag the animal and put them in a freezer when no heart beat can be found.
Now in a rare circumstance, i've heard of an occasional animal going into a coma as a result of the euthanazia solution and not actually passing on- so they will be mistakenly put in the freezer and survive(aweful, i know!). That can truely be an accident just depending. Having this happen 5 times at that shelter however is NOT an accident- they're not doing something correctly. My guess is they're not weighing the animals and getting an exact weight (necessary to administer the euthanazia solution properly).