Bukkit

I posted this in the comments, so I'm just going to quote directly from it:

"Yeah, funny story. When I made a sign for a turnstile, it put 20 characters on one of the lines. Then I had to use MCEdit to get rid of the sign. (When the client loads a chunk, containing a sign with too many characters, the client disconnects.)

EDIT: Apparently, if your economy system (I use iConomy) supports the "cent" system, and your currency has a relatively long name, this plugin will try to use that name, resulting in the sign issue.

For instance, if I used "Dollars" and "Cents" as my currency's name, and I sent the price of a turnstile to 10.05, then set a sign to display the price, it would display on a single line: "10 Dollars, 5 Cents" which is clearly too long.

Instead, you should just display it as "10.5 Dollars" (using this same scenario), AND add a failsafe to destroy the sign instantly if the plugin does indeed try to put too many characters on the same line."