Current bitcoin-master on testnet, `maxconnections=8` in the config file. Running in VM that doesn't have public IP and is not reachable from the Internet.

Wait until you have 8 peers.

```
telnet localhost 18333
```

It will result in:

```
Assertion failed: (nMaxInbound > 0), function AcceptConnection, file net.cpp, line 973.
```

First found in https://github.com/bitcoin/bitcoin/pull/8996, but can be reproduced on master.

{
    {
        LOCK(cs_vAddedNodes);
        vAddedNodes = mapMultiArgs["-addnode"];
    }
    for (unsigned int i = 0; true; i++)
    {
        std::vector<AddedNodeInfo> vInfo = GetAddedNodeInfo();
        for (const AddedNodeInfo& info : vInfo) {
            if (!info.fConnected) {
                CSemaphoreGrant grant(*semOutbound);
                
                
                CService service(LookupNumeric(info.strAddedNode.c_str(), Params().GetDefaultPort()));
                OpenNetworkConnection(CAddress(service, NODE_NONE), false, &grant, info.strAddedNode.c_str(), false);
                MilliSleep(500);
            }
        }
        MilliSleep(120000); 
    }
}
bool CConnman::OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound, const char *pszDest, bool fOneShot, bool fFeeler)