I seem to have provoked some sort of timing issue when using this on a machine with source on an SSD-dics, but liferay running of a non-SSD.
Caused by: java.util.concurrent.ExecutionException: org.jruby.embed.EvalFailedException: (Errno::EACCES) /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/work/Catalina/localhost/minecraftstuff-theme/sass/a0ae2da7aeaf9d37ea636d6ba73
fb7d7bcd66f08/custom.cssc
unlink at org/jruby/RubyFile.java:1801
_retrieve at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/cache_stores/filesystem.rb:25
retrieve at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/cache_stores/base.rb:63
retrieve at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/cache_stores/chain.rb:25
each at org/jruby/RubyArray.java:1612
each_with_index at org/jruby/RubyEnumerable.java:951
retrieve at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/cache_stores/chain.rb:24
_to_tree at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/engine.rb:333
to_tree at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/engine.rb:272
visit_import at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/perform.rb:225
send at org/jruby/RubyKernel.java:2097
visit at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/base.rb:37
visit at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/perform.rb:100
visit_children at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/base.rb:53
collect at org/jruby/RubyArray.java:2318
visit_children at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/base.rb:53
visit_children at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/perform.rb:109
with_environment at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/perform.rb:121
visit_children at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/perform.rb:108
visit at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/base.rb:37
visit_root at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/perform.rb:128
send at org/jruby/RubyKernel.java:2097
visit at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/base.rb:37
visit at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/perform.rb:100
send at org/jruby/RubyKernel.java:2097
visit at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/visitors/perform.rb:7
render at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/tree/root_node.rb:20
_render at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/engine.rb:315
render at /storage/liferay/liferay-portal-6.2-ce-ga4/tomcat-7.0.42/temp/liferay/ruby/gems/sass-3.2.12/lib/sass/engine.rb:262
(root) at <script>:48
Without knowing anything about the sass-engine, it seems to me like it is trying to delete a file that some other process still has a lock on?
Btw. Error messages are in liferay log, not gulp log.