Running an 18w11a server. I regenerate the world with each snapshot. - In the latest snapshot I've gotten this error twice now. Both times crashing the server. Both times it was a minecraft:drowned entity (not the same id) causing the error.
–
Ticking entity

java.lang.ClassCastException: afn cannot be cast to afj

– Crash Report
---- Minecraft Crash Report ----
// Shall we play a game?

Time: 3/16/18 12:41 PM
Description: Ticking entity

java.lang.ClassCastException: afn cannot be cast to afj
at adb.a(SourceFile:41)
at acz.a(SourceFile:22)
at adl.a(SourceFile:94)
at abw.cL(SourceFile:644)
at abv.l(SourceFile:1895)
at abw.l(SourceFile:504)
at ajs.l(SourceFile:33)
at aki.l(SourceFile:257)
at abv.S_(SourceFile:1750)
at abw.S_(SourceFile:284)
at ajs.S_(SourceFile:38)
at aki.S_(SourceFile:232)
at ajg.S_(SourceFile:252)
at aug.a(SourceFile:1617)
at se.a(SourceFile:705)
at aug.h(SourceFile:1593)
at aug.p_(SourceFile:1475)
at se.p_(SourceFile:538)
at net.minecraft.server.MinecraftServer.w(SourceFile:702)
at rp.w(SourceFile:354)
at net.minecraft.server.MinecraftServer.v(SourceFile:627)
at net.minecraft.server.MinecraftServer.run(SourceFile:532)
at java.lang.Thread.run(Unknown Source)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– Head –
Thread: Server thread
Stacktrace:
at adb.a(SourceFile:41)
at acz.a(SourceFile:22)
at adl.a(SourceFile:94)
at abw.cL(SourceFile:644)
at abv.l(SourceFile:1895)
at abw.l(SourceFile:504)
at ajs.l(SourceFile:33)
at aki.l(SourceFile:257)
at abv.S_(SourceFile:1750)
at abw.S_(SourceFile:284)
at ajs.S_(SourceFile:38)
at aki.S_(SourceFile:232)
at ajg.S_(SourceFile:252)
at aug.a(SourceFile:1617)
at se.a(SourceFile:705)
at aug.h(SourceFile:1593)

– Entity being ticked –
Details:
Entity Type: minecraft:drowned (ajg)
Entity ID: 83081
Entity Name: Drowned
Entity's Exact location: 29942.30, 60.72, -90.97
Entity's Block location: World: (29942,60,-91), Chunk: (at 6,3,5 in 1871,-6; contains blocks 29936,0,-96 to 29951,255,-81), Region: (58,-1; contains chunks 1856,-32 to 1887,-1, blocks 29696,0,-512 to 30207,255,-1)
Entity's Momentum: 0.00, 0.00, -0.01
Entity's Passengers: []
Entity's Vehicle: ~_ERROR~ NullPointerException: null
Stacktrace:
at aug.p_(SourceFile:1475)
at se.p_(SourceFile:538)

– Affected level –
Details:
Level name: world
All players: 1 total; [sg['___Assassin___'/79436, l='world', x=29907.07, y=61.09, z=-131.84]]
Chunk stats: ServerChunkCache: 555 Drop: 0
Level seed: 1505535
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: World: (192,64,-12), Chunk: (at 0,4,4 in 12,-1; contains blocks 192,0,-16 to 207,255,-1), Region: (0,-1; contains chunks 0,-32 to 31,-1, blocks 0,0,-512 to 511,255,-1)
Level time: 4410938 game time, 4750061 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 100414 (now: false), thunder time: 69453 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
at net.minecraft.server.MinecraftServer.w(SourceFile:702)
at rp.w(SourceFile:354)
at net.minecraft.server.MinecraftServer.v(SourceFile:627)
at net.minecraft.server.MinecraftServer.run(SourceFile:532)
at java.lang.Thread.run(Unknown Source)

– System Details –
Details:
Minecraft Version: 18w11a
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.8.0_161, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 759039200 bytes (723 MB) / 1853358080 bytes (1767 MB) up to 3808428032 bytes (3632 MB)
JVM Flags: 0 total;
Profiler Position: N/A (disabled)
Player Count: 1 / 100; [sg['___Assassin___'/79436, l='world', x=29907.07, y=61.09, z=-131.84]]
Data Packs: vanilla, file/veritas0923 (incompatible)
Is Modded: Unknown (can't tell)
Type: Dedicated Server (map_server.txt)