Steps to reproduce:

• log on to Minecraft server
• Make awesome contraption using tripwire
• Test it and have fun for a session (works perfectly)
• log off and have dinner
• Log back on
• watch all the horses fall out of your horse dispenser because the tripwire is no longer detecting the horses
• Walk into the tripwire to see if it works when you walk through it (it doesn't)
• Destroy the tripwire and place again.
• Contraption works again.
• have fun playing with the horse dispenser
• Log off
• Log back on.
• watch all the horses fall out of your horse dispenser because the tripwire is no longer detecting the horses