Random signs at different locations sometimes just get displayed too dark. Happens in Vanilla singleplayer as well as on my Bukkit Server.
Replacing the sign or changing the light level does not fix the issue.
Additionally, when the HUD is hidden (F1 mode), ALL signs are displayed too dark.
This wouldn't really be a problem (besides looking weird), but when using colored text, you can't read some colors.

Yes, I know this is a duplicate of MC-33304, but since that issue was marked as resolved (which it isn't) i added this bug report.