Bug actually occurred in MC 1.7.2, which JIRA no longer offers as a product choice. Plain vanilla, no mods.

Situation:
3 licensed MC users in household
Playing a world we've had for 6 weeks
World runs on Player 1's laptop; opened to LAN for others in household
Player 2 typically connects from different laptop in household

Occurrence:
All players completely exit MC
Player 2 starts MC on Player 1's laptop,
Logs in with Player 2 account
Opens desired world locally
No other players

Result:
Player 2 no longer has his prior inventory, XP, or location.
Instead Player 2 has a copy of Player 1's inventory,
has the same XP level as Player 1,
and is in the exact location where Player 1 was when he exited earlier.
When Player 1 re-joins later, his inventory, XP, and location are unchanged - nothing appears to have happened to Player 1.
But Player 2 lost 20 levels of XP and a diamond pickaxe in the deal, not to mention finding himself in an unfamiliar location. Waaaa!

Fortunately this sucky bug has not re-occurred in the day of play that has followed.