svnmerge croaks on mysterious conflict

We run svn 1.6.x on both server and client. I branched, worked for several weeks, downmerged a few times, and then set out to upmerge back. svnmerge.py, as logged below, splits the commits into two sets, and on the second set, encounters a (false?) tree conflict, and then gets a subversion error and implodes (see the bottom of the log).

I'm loathe to commit this in two parts, since the intermediate state was not really working, so manually running the first revision set and then the second is not something I want to do.

property 'svnmerge-integrated' set on '.'

--- Merging r5171 through r6286 into '.':
U    java/buildtools/src/main/resources/checkstyle.xml
U    java/buildtools/pom.xml
U    java/com.basistech.rex2009.common/pom.xml
U    java/tokenize_lines/pom.xml
U    java/runtime/pom.xml
U    java/ldctools/pom.xml
U    java/models/pom.xml
U    java/gazdecode/pom.xml
U    java/annotatortools/pom.xml
Skipped 'java/pom-version.pl'
U    java/common/pom.xml
U    java/parent/pom.xml
U    java/pom.xml
A    java/ap-segmentation
Skipped 'java/ap-segmentation/TODO'
Skipped 'java/ap-segmentation/notes.txt'
A    java/ap-segmentation/scripts
Skipped 'java/ap-segmentation/scripts/prune-nbest.perl'
Skipped 'java/ap-segmentation/scripts/prune-threshold.perl'
A    java/ap-segmentation/src
A    java/ap-segmentation/src/test
A    java/ap-segmentation/src/test/java
A    java/ap-segmentation/src/test/java/com
A    java/ap-segmentation/src/test/java/com/basistech
A    java/ap-segmentation/src/test/java/com/basistech/seg
Skipped 'java/ap-segmentation/src/test/java/com/basistech/seg/SymbolsTest.java'
Skipped 'java/ap-segmentation/src/test/java/com/basistech/seg/FeatureVectorTest.java'
A    java/ap-segmentation/src/test/resources
A    java/ap-segmentation/src/main
A    java/ap-segmentation/src/main/java
A    java/ap-segmentation/src/main/java/com
A    java/ap-segmentation/src/main/java/com/basistech
A    java/ap-segmentation/src/main/java/com/basistech/seg
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/AveragedPerceptron.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/Decode.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/Trainer.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/Decoder.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/FeatureSet.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/Symbols.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/Feature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/SegFeatures.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/Train.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/FeatureVector.java'
A    java/ap-segmentation/src/main/java/com/basistech/seg/features
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/LastLengthFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/ConsecStartCharsFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/FirstLengthFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/WordFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/CharBeforeWordFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/CharBigramIntraSegFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/FirstLastCharsFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/WordBigramFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/WordLengthAndPreviousFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/SingleCharFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/WordLengthAndNextFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/CharBigramInterSegFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/ConsecEndCharsFeature.java'
A    java/ap-segmentation/src/main/resources
Skipped 'java/ap-segmentation/pom.xml'
U    java/maven-rex2009task-plugin/pom.xml
U    java/corpus-tasks/pom.xml
U    java/ama/pom.xml
U    java/rlp-bl-annotate/pom.xml
U    java/ape/pom.xml
A    scripts
Skipped 'scripts/score'
Skipped 'scripts/mwseg.pl'
Skipped 'scripts/pipelines.mk'
A    data
A    data/scripts
Skipped 'data/scripts/cleanup-whitespace.perl'
Skipped 'data/Makefile'
Summary of conflicts:
  Skipped paths: 36

property 'svnmerge-integrated' set on '.'

--- Merging r6297 through r6302 into '.':
   C java/common/src/main/java/com/basistech/perceptron
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/Decoder.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/LastLengthFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/ConsecStartCharsFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/FirstLengthFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/WordFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/CharBeforeWordFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/CharBigramIntraSegFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/FirstLastCharsFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/WordBigramFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/WordLengthAndPreviousFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/SingleCharFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/WordLengthAndNextFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/CharBigramInterSegFeature.java'
Skipped 'java/ap-segmentation/src/main/java/com/basistech/seg/features/ConsecEndCharsFeature.java'
Summary of conflicts:
  Tree conflicts: 1
  Skipped paths: 14

property 'svnmerge-integrated' set on '.'

svnmerge: command execution failed (exit code: 1)
svn --non-interactive merge --force -r 6341:6426 http://svn.basistech.net/engineering/rex2009/branches/worm .
svn: Attempt to add tree conflict that already exists
svn: Error reading spooled REPORT request response

The explanation has emerged, and I guess this might be interesting to someone else.

Never, ever, forget to commit a run of svnmerge.py before doing something else. Combining a merge with other edits is a recipe for a disaster, and the disaster is what you see in the question.