I noticed this bug again lately. Sorry can not reproduce right now but I did indeed download several small areas. The cursor locks as arrow pointing NW without rectangle. Switching modes usually unlocks it again.
Repository Root: http://josm.openstreetmap.de/svn
Build-Date: 2012-11-20 02:31:11
Last Changed Author: simon04
Revision: 5593
Repository UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
URL: http://josm.openstreetmap.de/svn/trunk
Last Changed Date: 2012-11-19 11:36:21 +0100 (Mon, 19 Nov 2012)
Last Changed Rev: 5593
Identification: JOSM/1.5 (5593 en)
Memory Usage: 433 MB / 643 MB (228 MB allocated, but free)
Java version: 1.6.0_24, Sun Microsystems Inc., OpenJDK 64-Bit Server VM
Operating system: Linux
Dataset consistency test: No problems found