Hi Carnac,
Typically, the symbol for "sickness" is saying the work plane has lost a
reference to something that was used to define it. For example, a workplane
is created using some upstream edge or feature. That edge or feature is
later altered significantly or removed. At the time this happens there is
usually an error like "problems occurred - could not build this plane" and
the error is accepted. This symbol in front of a modeling feature usually
means a similar thing has happened. Redefining the "sick" feature usually
cures the problem. However, there are different levels of "sickness" and in
some cases the feature needs to be deleted and recreated.
I hope this helps,
Harold Lee
Inventor QA