I've been bitten by this particularly wicked bug; it's happened to me twice, at the most inopportune times (e.g., when you have Duriel down to 10% health).
The easiest solution is to set USB Overdrive to disable itself for Diablo II (thru the control panel).
D2 can see second mouse buttons without USB Overdrive, so you lose nothing by disabling it for that game.
(although, if you have a 4-button mouse, that's a different story)
Also make sure you have the latest USBO; an update was posted eariler this week.