I'm trying to find the source of some drops, but I can't find any documentation on what these mean from the output of show drops:
UNKNOWN_L2_ON_L3_DISCARD: Does this mean that it saw a tagged come in on the interface when the interface is not configured as a trunk?
L3_NOT_MYMAC: Does this mean that the packet which arrived on the interface had the incoming interface's IP as the destination IP, but not the incoming interface's MAC as the destination MAC?
The interface reporting these errors is connected to a public Internet Exchange, so it's possible that these errors are caused by the IX being laxed about what type of traffic is allowed to hit the switch fabric.
the article that explains the np drop counters should be back online shortly. due to an unfortunate mishap it was taken offline.
also the show controller np description will provide similar detail (release dependent).
the unknown l2 on l3 discard is somewhat nasty and besdies the one that rivalino highlighted there are a couple of tother cases.
you can always capture the packets with the procedure to capture drop packets by setting a trap on that counter to see what is going on. generally it comes down to some interpretation or processing error between L2 and L3 domains. (like BD's, BVI's, routing and switching in the same feature path).