In Ubuntu Gutsy, if you have the "Crux" visual theme selected, OpenOffice Calc
locks up 100% of the time when you work on ANY spreadsheet and select the
"Format--Cells" option from the menu.  If Ubuntu Visual Effects are turned off,
the app simply freezes.  If Visual Effects are turned on, the window goes gray
first, but is also frozen otherwise.

I've only verified this behavior on Ubuntu Gutsy x86_64, with OpenOffice 2.3. 
At least with the default Gutsy themes I believe it only happens with Crux,
although that may merit some additional testing (I personally tried the Human,
Clearlooks, Mist, Glossy, Glider themes though, and all work fine).  

Since I know for a fact that changing themes fixes the problem, the criticality
of this isn't that high, but I'm thinking a lot of end-users won't even think to
TRY changing the theme for a problem with a spreadsheet.  And I think a
disproportionate amount of people use Crux, because it has a few extra visual
touches (most people use the default Human theme, of course, but Crux has got to
be #2).