Errores

After the paramedics arrive and Bree is conscious again, you can see in a shot that all the blood surrounding the hole in the fiberglass where they escaped from is gone.

The "shot" Bree supposedly "needs," and with which her sister injects her toward the end of the film, is insulin (via an insulin pen). Insulin *lowers* blood sugar. The film essentially gets every aspect of type 1 diabetes mellitus incorrect.

The shot used to revive Bree in the end is an red/orange glucagon pen.

The sisters recover the engagement ring but never think to use it to cut a hole in the fiberglass cover. If the stone in the ring is a diamond, it would cut through fiberglass.

After the pool heater is shut off it takes only minutes before the water starts feel cold. In reality a large pool like this with thousand of liters of water (in a heated room no less) would take several days with no heat before the temperature drops enough to even notice.

One of the sisters tried to pull the metal cover from the bottom of the pool. As she is pulling on the cover, she is also standing on top of it.

After Clara discovered that the code for the swimming pool cover has been changed, she could have use the sister's mobile phone to SMS the fiancee for help and that would have not revealed herself.

After their first attempt to dismantle the grate failed, they could have easily undress and use their swimming suit to pull the grate, then use it to smash the fiberglass cover.