We have a dog who is on pimobendan, and our other dog who is about 13 pounds accidentally got .625 mg of it.
Will he be okay??
You should always call your vet as soon as possible when something like this happens. By the time someone on the internet answers, if it was a problem it will likely have resulted in serious effect by now.
That being said, 0.625 mg of pimobendan is even a low dose for a 13 lb dog (roughly 0.1 mg/kg). Typical therapeutic doses range from 0.2 to 0.5 mg/kg per day (per Plumbs Veterinary Drugs).
Toxicity is therefore not a concern at such low dose.
In all likelihood, it will not be a problem. Pimobendan works to increase heart contractility, and in a healthy dog a single low dose will not be a problem. The concern comes if your dog has heart disease or other conditions which may make reaction to the drug less predictable.
Close monitoring for 24 hours is a reasonable option in such a case; if any doubts or concerns then immediate evaluation by a veterinarian is needed.