Is it a crime for a repo man to accidentally repo the wrong car?
Not unless the car was retained after the accidental repossession was discovered, and then, only by the person retaining it (as the repo man may have turned over the car to the creditor whose loan on a similar car is in default).
Generally speaking, taking property of another with an intent to permanently deprive the owner of the property of it is a crime only if one knows that the property is the property of another.
For example, if two people leave black umbrellas in an entry room and someone accidentally leaves with the wrong one, the taking of the wrong umbrella is not a crime.
Whether the repo man's assertion that he accidentally took the wrong car is credible is a question of fact to be determined at trial, if the prosecution doubts him. If he was supposed to repossess a 1936 Ford and he repossessed a 2021 Tesla, the repo man is probably going to lose and be convicted of theft. If he was supposed to repossess a white 2021 Tesla and he repossessed a different white 2021 Tesla in the same neighborhood with a license plate from the same state as the one he was supposed to repossess, he has a very good chance of prevailing.
However, once someone learns that they have taken the wrong property, they have a duty to return the property promptly to the owner upon request, and probably, to notify the owner (if the owner can be determined) and the authorities who were informed that a different vehicle was taken, promptly. Otherwise, the originally good faith mistake becomes theft.
If the repo man's explanation is convincing, he is not likely to be charged with theft, even though no special law applies. What makes the repo man special is that he did have permission from the secured car loan creditor to repossess it due to the secured car loan debtor's default by the Uniform Commercial Code. If he had taken the right car without a breach of the peace, the Uniform Commercial Code would have absolved him of liability and given him legal permission to do so.
If the repo had been of the right car, the creditor would have had a duty to promptly return the personal possession in the car in which it did not have a lien to the rightful owner. This conclusion doesn't change when the repo man accidentally takes the wrong car.
While the repo man's mistake was not knowing or intentional, it was probably negligent to repossess the car without carefully confirming the VIN number and license plate to make sure that he was repossessing the correct car. As a result, the car own probably has a claim against both the repo man (whose negligence caused the wrongful repossession) and the creditor (for whom he was acting as an agent to repossess the car) for any damages caused to the owner of the wrongfully repossessed car, including damages to the vehicle and damages from loss of use of the car and possibly damages for emotional distress caused by thinking that his car had been stolen or by missing a non-economic opportunity that he could have had if the car had not been wrongfully taken (e.g. if this caused the car owner to miss the funeral of the car owner's father).
The creditor and the repo man probably have insurance policies in place that cover legal defense of such claims and also economic settlements or money judgments entered on that kind of claim.