At some point during connection, the command to /summon {name|target} stops working. I think if 2 people log into the same zone and try it, it'll work once. But if zoning or /reloading occurs, /summon stops working. No error message returned about not finding the spawn, or whatever.

Dunno what the exact condition is, but when I demand Scatman's attention NOW I can no longer /summon him to do my bidding.

Does this happen when the player is in another zone and you try to summon them?

After we get into the state where /summon {name} no longer works, you cannot even summon the player if they are in the SAME zone. But yes, either way. I can try running some tests today to determine when it actually breaks down.