Mobs Frozen in Line

This looks like a server-side crash.

We do not have enough information to find the cause of this issue.

Please attach the crash report found in [minecraft/crash-reports/crash-<DATE>-server.txt|https://minecrafthopper.net/help/finding-minecraft-data-folder/] here.
If you cannot find a crash report, please attach the full launcher log found in [minecraft/launcher_log.txt|https://minecrafthopper.net/help/finding-minecraft-data-folder/].

_{This issue is being temporarily closed as Awaiting Response. Once the requested information has been delivered, the report will be reopened automatically.}

Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki

This looks like a server-side crash.

We do not have enough information to find the cause of this issue.

Please attach the crash report found in [minecraft/crash-reports/crash-<DATE>-server.txt|https://minecrafthopper.net/help/finding-minecraft-data-folder/] here.
If you cannot find a crash report, please attach the full launcher log found in [minecraft/launcher_log.txt|https://minecrafthopper.net/help/finding-minecraft-data-folder/].

_{This issue is being temporarily closed as Awaiting Response. Once the requested information has been delivered, the report will be reopened automatically.}

Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki

I wasn't on a server, this was on a singleplayer world

That's fine, but all singleplayer worlds are run on an internal server since Minecraft 1.3 🙂 

Yup, even if you're playing single player, a server still runs in the background

Yup, even if you're playing single player, a server still runs in the background