Wireless Access

What could cause a wireless client to show an MCS index of 0 and have a speed of 300 Mbps?

I'm checking this on a Mac where I hold the option key and click the wireless chooser.
#ALE

 
You should probably do a wireless capture , when you hold the option key on a Mac the device stops scanning so that value may not be accurate
This tool can capture raw frames, show roaming events
/System/Library/CoreServices/Wi-Fi Diagnostics.app
 
You can also run this commands
 
 
MacbookPro:~ vfabian$ sudo ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport /usr/sbin/airport
Password:
ln: /usr/sbin/airport: File exists
MacbookPro:~ vfabian$  cd /usr/sbin
SMacbookPro:sbin vfabian$
MacbookPro:sbin vfabian$ sudo ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport
ln: ./airport: File exists
MacbookPro:sbin vfabian$
SMacbookPro:sbin vfabian$
MacbookPro:sbin vfabian$ airport -I
     agrCtlRSSI: -50 ------>>>>> Wireless Signal
     agrExtRSSI: 0
    agrCtlNoise: -91 -
    agrExtNoise: 0
          state: running
        op mode: station
     lastTxRate: 78
        maxRate: 144 
lastAssocStatus: 0
    802.11 auth: open
      link auth: wpa2 
          BSSID: 0:24:6c:2c:50:3b
           SSID: home_secure
            MCS: 12 
        channel: 149

What version of OS X? I vaguely remember this being a bug.