I've just created a new tag on SO so I tried to create a tag wiki for it. Since I'm not an expert on the topic I wrote an excerpt only, but when I tried to submit it I got the 'oops' cat error page.
I don't have enough rep / votes on the tag to really edit it so my submission was a proposed edit only. I tried this for an existing tag too, and it also crashed. I then tried filling in a tag wiki body too - in this case just a link to Wikipedia - and it saved correctly.
- pick a tag with no existing wiki
- edit tag info
- write an excerpt only + submit - crash
- go back (you may need to reload the page: the save button didn't re-enable) and write a body for the wiki as well as the excerpt + submit - saves OK
I'm not clear whether this depends on whether you have permission to edit the wiki for the specific tag - there aren't any tags I do have enough votes to edit regardless, nor do I have enough total rep to edit everywhere.