As the comment in plugin.sh already notes:
# Look up warning environment variables in the following order:
# $1 = field name
# $2 = optional override of environment variable name
#
# Hmm, this first looks for field_warning, then $2 then warning. Not the
# order one expects.
I think the function should look for $2, then $1_warning, then warning, as the comment suggests.
I understand that this would cause some backwards incompatibility, but after doing some grepping on plugins included with munin 1.4.5, I find no plugins that use the second argument when calling this function.
This mean that there is no impact on existing (bundled) plugins. Please consider this small incompatibility change for the major version bump to Munin 2.0.
