Discovered today that Puppet arrays have a built-in flatten method (which is actually provided by the underlying Ruby array).
This can make dealing with potentially nested arrays in ERB templates much easier.

The following example is from the ERB documentation:

# Peers
<% [@peers].flatten.each do |peer| -%>
peer <%= peer %>
<% end -%>

This allows for nice flexibility as @peers can now be either a single value, an array, or a nested array and all are handled in the same way without needing to write complicated if/else statements.