I started a reversion as described below and the system threw this error before returning to user control stating a coding error and to send this to you.

[neile.oxcab@…]

Revision: 8677
Repository Root: http://josm.openstreetmap.de/svn
Relative URL: ^/trunk
Last Changed Author: Don-vip
Last Changed Date: 2015-08-21 22:18:03 +0200 (Fri, 21 Aug 2015)
Build-Date: 2015-08-21 23:32:08
URL: http://josm.openstreetmap.de/svn/trunk
Repository UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last Changed Rev: 8677

Identification: JOSM/1.5 (8677 en_GB) Windows 7 32-Bit
Memory Usage: 159 MB / 247 MB (56 MB allocated, but free)
Java version: 1.8.0_25, Oracle Corporation, Java HotSpot(TM) Client VM
VM arguments: [-Djava.security.manager, -Djava.security.policy=file:C:\Program Files\Java\jre1.8.0_25\lib\security\javaws.policy, -DtrustProxy=true, -Djnlpx.home=<java.home>\bin, -Djnlpx.origFilenameArg=C:\Users\NeilE\AppData\LocalLow\Sun\Java\Deployment\cache\6.0\56\1ee8cfb8-7693477c, -Djnlpx.remove=false, -Djava.util.Arrays.useLegacyMergeSort=true, -Djnlpx.splashport=50097, -Djnlp.application.href=https://josm.openstreetmap.de/download/josm.jnlp, -Djnlpx.jvm=<java.home>\bin\javaw.exe, -Djnlpx.vmargs=LURqYXZhLnV0aWwuQXJyYXlzLnVzZUxlZ2FjeU1lcmdlU29ydD10cnVlAC1Eam5scC5hcHBsaWNhdGlvbi5ocmVmPWh0dHBzOi8vam9zbS5vcGVuc3RyZWV0bWFwLmRlL2Rvd25sb2FkL2pvc20uam5scAA=]
Dataset consistency test: No problems found

Plugins:
- ColorPlugin (1414145445)
- DxfImport (UNKNOWN)
- ImportImagePlugin (31241)
- OpeningHoursEditor (31241)
- PicLayer (31241)
- RoadSigns (31241)
- SeaMapEditor (31241)
- TombPlugin (44)
- buildings_tools (31361)
- continuosDownload (1413902943)
- fieldpapers (v0.3.0)
- geochat (31241)
- geotools (31126)
- graphview (31241)
- importvec (31241)
- jts (31126)
- junctionchecking (31241)
- lakewalker (31241)
- log4j (31231)
- mapdust (31241)
- opendata (31241)
- osmarender (31241)
- pdfimport (31241)
- plastic_laf (31126)
- pointInfo (31241)
- poly (31241)
- print (31329)
- proj4j (31353)
- public_transport (31241)
- public_transport_layer (31220)
- reverter (31241)
- routes (31241)
- tageditor (31241)
- terracer (31241)
- turnlanes (31241)
- turnrestrictions (31241)
- undelete (31241)
- utilsplugin2 (31463)
- waydownloader (31241)
- waypoint_search (31241)
- wms-turbo-challenge2 (31308)

Last errors/warnings:
- W: Failed to parse Mappaint styles from 'C:\Users\NeilE\Documents\New file organisation\projects\Mapping\system support\mapcss\openstreetbrowser2\OpenStreetBrowser-basemap-master\water.mapcss'. Error was: Unknown MapCSS base selector line
- E: java.lang.RuntimeException: Unknown MapCSS base selector line
- W: New conflict: Conflict [my={Node id=2691544784 version=2 D }, their={Node id=2691544784 version=0 IV lat=51.749989,lon=-1.2430804}]
- W: New conflict: Conflict [my={Node id=2691544784 version=2 D }, their={Node id=2691544784 version=0 IV lat=51.749989,lon=-1.2430804}]
- E: java.lang.IllegalStateException: already registered a conflict for primitive '{Node id=2691544784 version=2 D }'

java.lang.IllegalStateException: already registered a conflict for primitive '{Node id=2691544784 version=2 D }'
	at org.openstreetmap.josm.data.conflict.ConflictCollection.addConflict(ConflictCollection.java:108)
	at org.openstreetmap.josm.data.conflict.ConflictCollection.add(ConflictCollection.java:123)
	at reverter.DataSetCommandMerger.mergeWay(DataSetCommandMerger.java:123)
	at reverter.DataSetCommandMerger.merge(DataSetCommandMerger.java:173)
	at reverter.DataSetCommandMerger.<init>(DataSetCommandMerger.java:44)
	at reverter.ChangesetReverter.getCommands(ChangesetReverter.java:344)
	at reverter.RevertChangesetTask.realRun(RevertChangesetTask.java:104)
	at org.openstreetmap.josm.gui.PleaseWaitRunnable.doRealRun(PleaseWaitRunnable.java:93)
	at org.openstreetmap.josm.gui.PleaseWaitRunnable.run(PleaseWaitRunnable.java:161)
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
	at java.util.concurrent.FutureTask.run(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)

[simon04]

Closed as duplicate of #11811.