Bukkit

What steps will reproduce the problem?
Load the server
Place a chest then sign to set up shop
Click the shop sign with the item i wish to sell

What is the expected output? What do you see instead?
Expected: The shop sign will change showing that the shop was set up and that people can now buy from it. What happens is that the item shows up above the chest like its meant to but the sign does not update and the shop dose not activate. The sign will now break as it thinks you have yet to set up a shop and will keep asking you to click with an item to set the shop up. clicking with an item will make another item show up above the chest, this can create a lot of lag as you can put as many items as you like above that one spot.

What version of the product are you using?
Shop 1.4.8
Minecraft 1.7.9
Spigot 1470

Do you have an error log of what happened?
http://pastebin.com/a3sSB28k

Please provide any additional information below.

Found out what was creating the error, ClearLag was conflicting. Not sure how it was but when I remove that plugin Shop started working again.