If you have smart variable matcher for environment, add another matcher for FQDN, and try to edit host afterwards, it will fail with the following error:

ArgumentError
comparison of Array with Array failed
app/services/classification/base.rb:41:in `block in values_hash'
app/services/classification/base.rb:39:in `values_hash'
app/services/classification/base.rb:17:in `inherited_values'
app/views/puppetclasses/_class_parameters.html.erb:2:in `_6bd4e894887042c12c1c876d3bbd59b5'
app/views/puppetclasses/_classes_parameters.html.erb:12:in `_efecede07e99fec46c2aace6cca47e7e'
app/views/hosts/_form.html.erb:96:in `block (2 levels) in _d083b3a0d59e018a9d7bfcdea74457b4'
app/helpers/layout_helper.rb:273:in `form_for'
app/views/hosts/_form.html.erb:8:in `block in _d083b3a0d59e018a9d7bfcdea74457b4'
app/models/taxonomy.rb:61:in `block (2 levels) in as_taxonomy'
app/models/concerns/foreman/thread_session.rb:147:in `as_location'
app/models/taxonomy.rb:60:in `block in as_taxonomy'
app/models/concerns/foreman/thread_session.rb:112:in `as_org'
app/models/taxonomy.rb:59:in `as_taxonomy'
app/views/hosts/_form.html.erb:6:in `_d083b3a0d59e018a9d7bfcdea74457b4'
app/views/hosts/edit.html.erb:9:in `_ad432273d701fcf94b0032cbe38693a1'
app/controllers/concerns/application_shared.rb:13:in `set_timezone'
app/models/concerns/foreman/thread_session.rb:32:in `clear_thread'
lib/middleware/catch_json_parse_errors.rb:9:in `call'

Moreover, the following errors will appear in Puppet reports:

Could not retrieve catalog from remote server: Error 400 on SERVER: Failed when searching for node test.node.local: Failed to find test.node.local via exec: Execution of '/etc/puppet/node.rb test.node.local' returned 1:

In my case, I have a Puppet class variable "deploy_keys" and a smart variable matcher: "environment = staging".
Later on, I add matcher "fqdn = test.node.local" and voila, errors start to appear.

Foreman version: 1.8.2