I guess it is fixed at [Bug #14553] r63040.
(And it also needs [Bug #14660] r63103 to pass all tests, but although it is not directly related to this issue.)
Would you please retry with latest trunk ruby?
An Execution result of my environment is here.
$ bundle exec bin/testunit && ruby -v
/work/bootsnap/.bundle/ruby/2.6.0/gems/mocha-1.5.0/lib/mocha/return_values.rb:19: warning: mismatched indentations at 'when' with 'case' at 18
/work/bootsnap/.bundle/ruby/2.6.0/gems/mocha-1.5.0/lib/mocha/return_values.rb:20: warning: mismatched indentations at 'when' with 'case' at 18
/work/bootsnap/.bundle/ruby/2.6.0/gems/mocha-1.5.0/lib/mocha/return_values.rb:21: warning: mismatched indentations at 'else' with 'case' at 18
/work/bootsnap/.bundle/ruby/2.6.0/gems/mocha-1.5.0/lib/mocha/yield_parameters.rb:15: warning: mismatched indentations at 'when' with 'case' at 14
/work/bootsnap/.bundle/ruby/2.6.0/gems/mocha-1.5.0/lib/mocha/yield_parameters.rb:16: warning: mismatched indentations at 'when' with 'case' at 14
/work/bootsnap/.bundle/ruby/2.6.0/gems/mocha-1.5.0/lib/mocha/yield_parameters.rb:17: warning: mismatched indentations at 'else' with 'case' at 14
/work/bootsnap/.bundle/ruby/2.6.0/gems/mocha-1.5.0/lib/mocha/cardinality.rb:23: warning: mismatched indentations at 'when' with 'case' at 22
/work/bootsnap/.bundle/ruby/2.6.0/gems/mocha-1.5.0/lib/mocha/cardinality.rb:24: warning: mismatched indentations at 'else' with 'case' at 22
/work/bootsnap/.bundle/ruby/2.6.0/gems/mocha-1.5.0/lib/mocha/cardinality.rb:82: warning: mismatched indentations at 'when' with 'case' at 81
/work/bootsnap/.bundle/ruby/2.6.0/gems/mocha-1.5.0/lib/mocha/cardinality.rb:83: warning: mismatched indentations at 'when' with 'case' at 81
/work/bootsnap/.bundle/ruby/2.6.0/gems/mocha-1.5.0/lib/mocha/cardinality.rb:84: warning: mismatched indentations at 'when' with 'case' at 81
/work/bootsnap/.bundle/ruby/2.6.0/gems/mocha-1.5.0/lib/mocha/cardinality.rb:85: warning: mismatched indentations at 'else' with 'case' at 81
Run options: --seed 31648
# Running:
.....................................................................................................................
Finished in 0.118636s, 986.2100 runs/s, 1711.1165 assertions/s.
117 runs, 203 assertions, 0 failures, 0 errors, 0 skips
ruby 2.6.0dev (2018-04-07 trunk 63107) [x86_64-linux]
Thanks for the update. This issue does not reproduce anymore using ruby 2.6.0dev (2018-04-07 trunk 63108) [x86_64-linux]
$ bundle exec bin/testunit
/home/yahonda/.rbenv/versions/2.6.0-dev/lib/ruby/gems/2.6.0/gems/mocha-1.4.0/lib/mocha/return_values.rb:19: warning: mismatched indentations at 'when' with 'case' at 18
/home/yahonda/.rbenv/versions/2.6.0-dev/lib/ruby/gems/2.6.0/gems/mocha-1.4.0/lib/mocha/return_values.rb:20: warning: mismatched indentations at 'when' with 'case' at 18
/home/yahonda/.rbenv/versions/2.6.0-dev/lib/ruby/gems/2.6.0/gems/mocha-1.4.0/lib/mocha/return_values.rb:21: warning: mismatched indentations at 'else' with 'case' at 18
/home/yahonda/.rbenv/versions/2.6.0-dev/lib/ruby/gems/2.6.0/gems/mocha-1.4.0/lib/mocha/yield_parameters.rb:15: warning: mismatched indentations at 'when' with 'case' at 14
/home/yahonda/.rbenv/versions/2.6.0-dev/lib/ruby/gems/2.6.0/gems/mocha-1.4.0/lib/mocha/yield_parameters.rb:16: warning: mismatched indentations at 'when' with 'case' at 14
/home/yahonda/.rbenv/versions/2.6.0-dev/lib/ruby/gems/2.6.0/gems/mocha-1.4.0/lib/mocha/yield_parameters.rb:17: warning: mismatched indentations at 'else' with 'case' at 14
/home/yahonda/.rbenv/versions/2.6.0-dev/lib/ruby/gems/2.6.0/gems/mocha-1.4.0/lib/mocha/cardinality.rb:23: warning: mismatched indentations at 'when' with 'case' at 22
/home/yahonda/.rbenv/versions/2.6.0-dev/lib/ruby/gems/2.6.0/gems/mocha-1.4.0/lib/mocha/cardinality.rb:24: warning: mismatched indentations at 'else' with 'case' at 22
/home/yahonda/.rbenv/versions/2.6.0-dev/lib/ruby/gems/2.6.0/gems/mocha-1.4.0/lib/mocha/cardinality.rb:82: warning: mismatched indentations at 'when' with 'case' at 81
/home/yahonda/.rbenv/versions/2.6.0-dev/lib/ruby/gems/2.6.0/gems/mocha-1.4.0/lib/mocha/cardinality.rb:83: warning: mismatched indentations at 'when' with 'case' at 81
/home/yahonda/.rbenv/versions/2.6.0-dev/lib/ruby/gems/2.6.0/gems/mocha-1.4.0/lib/mocha/cardinality.rb:84: warning: mismatched indentations at 'when' with 'case' at 81
/home/yahonda/.rbenv/versions/2.6.0-dev/lib/ruby/gems/2.6.0/gems/mocha-1.4.0/lib/mocha/cardinality.rb:85: warning: mismatched indentations at 'else' with 'case' at 81
Run options: --seed 25408
# Running:
.....................................................................................................................
Finished in 0.104083s, 1124.1033 runs/s, 1950.3672 assertions/s.
117 runs, 203 assertions, 0 failures, 0 errors, 0 skips
[yahonda@li1885-190 bootsnap (master)]$ ruby -v
ruby 2.6.0dev (2018-04-07 trunk 63108) [x86_64-linux]
$