Question: I have a custo workflow that creates an account and opportunities.

Sometimes I have this error: Account with Id = "xxxxxx" does not exist.

I don't know what's wrong in my code knowing that I find the account in the CRM.  

Here are the steps of my plugin code:

1. Find the account by num (if it doesn't exist, I create them)
2. Get the account = Account
3. Create an opportunity with Opportunity["parentaccountid"] = Account;
4. Error message !

Code:

```cs
//Get opportunity
Guid id = retrieveOpportunity<string>("opportunity", "new_numero", numero, service);
Entity eOpportunity;
if (id != Guid.Empty)
{
    eOpportunity = new Entity("opportunity", id);
}
else
{
    eOpportunity = new Entity("opportunity");
}

//Get account
EntityReference eAccount = retrieveAccount<string>(accountCode, "account", "new_code", service);
if (eAccount == null)
{
    eAccount = new Entity("account", "new_code", accountCode);
    eAccount["name"] = "name";
    UpsertRequest usMessage = new UpsertRequest()
    {
        Target = eAccount
    };
    //create account
    UpsertResponse usResponse = (UpsertResponse)this._service.Execute(usMessage);
    eOpportunity["parentaccountid"] = usResponse.Target;
}
else
{
    eOpportunity["parentaccountid"] = eAccount;
}

UpsertRequest req = new UpsertRequest()
{
    Target = eOpportunity
}; 
//upsert opportunity
UpsertResponse resp = (UpsertResponse)service.Execute(req);

if (resp.RecordCreated)
    tracer.Trace("New opportunity");
else
    tracer.Trace("Opportunity updated");

```

Sometimes there are several workflows that are started at the same time and that do the same thing (creating other opportunities)

Answer: 
You haven't shown us the entire plugin, so this is just a guess, but you're probably sharing your IOrganizationService at the class level, which is causing race conditions in your code, and one thread creates a new account in a different context, then its service gets overwritten by another thread, which is in a different database transaction that doesn't have the newly created account and it's erroring.

Don't share your IOrganziationService across threads!
Whenever you are trying to consume the created record in the same transaction, convert the plugin into Asynchronous mode - this will work.