I'm running WordPress 2.6.
When I create a new post and upload and include an image with a caption specified I get an error at the top of my posts right above the post body.
Warning: Compilation failed: characters with values > 255 are not yet supported in classes at offset 16 in /vservers/radianne/htdocs/wp-includes/shortcodes.php on line 213
The error only occurs when the image has a caption used for the ALT attribute of the IMG tag.
This was difficult to track down because there are bugs in updating the caption for images in the media library.
I enter a caption upon upload of a photo and include the image in my post. I get the error. Next I edit the photo properties and delete the caption, but the error remains because the ALT attribute is still showing the caption I deleted.
I upload an image without a caption (no error) and then edit the image to add a caption. There is no error because the ALT attribute is still blank.