Snapshot 13w16b, Snapshot 13w19a, Snapshot 13w21a, Snapshot 13w21b, Snapshot 13w22a, Snapshot 13w23b, Snapshot 13w24a, Snapshot 13w25a, Snapshot 13w26a
Windows 8 64-Bit
Mac OS X 10.8.4
Java 7 Update 25 (and Java 6 Update 51 for Mac OS X)
1. Get in minecart.
2. Click on it instead of LSHIFT.
As you see, it attempts to remove you from the minecart but cannot since the control for that was changed to LSHIFT.