If the player is near a spawner that creates polar bears, warnings are issued in the logbook every second.

[09:53:35] [Render thread/WARN]: Exception loading entity:
u: Loading entity NBT
at aqa.f(SourceFile:1663) ~[1.16.5.jar:?]
at aqe.a(SourceFile:473) ~[1.16.5.jar:?]
at aqe$$Lambda$4470/1016724550.accept(Unknown Source) ~[?:?]
at x.a(SourceFile:402) ~[1.16.5.jar:?]
at aqe.a(SourceFile:472) ~[1.16.5.jar:?]
at aqe.b(SourceFile:535) [1.16.5.jar:?]
at aqe.a(SourceFile:516) [1.16.5.jar:?]
at bqz.d(SourceFile:254) [1.16.5.jar:?]
at ecp.a(SourceFile:22) [1.16.5.jar:?]
at ecp.a(SourceFile:12) [1.16.5.jar:?]
at ecd.a(SourceFile:107) [1.16.5.jar:?]
at ecd.b(SourceFile:96) [1.16.5.jar:?]
at ecd$$Lambda$4920/1979264988.run(Unknown Source) [1.16.5.jar:?]
at ecd.a(SourceFile:128) [1.16.5.jar:?]
at ecd.a(SourceFile:96) [1.16.5.jar:?]
at eae.a(SourceFile:1260) [1.16.5.jar:?]
at dzz.a(SourceFile:727) [1.16.5.jar:?]
at dzz.a(SourceFile:546) [1.16.5.jar:?]
at djz.e(SourceFile:1048) [1.16.5.jar:?]
at djz.e(SourceFile:681) [1.16.5.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:215) [1.16.5.jar:?]
Caused by: java.lang.ClassCastException: dwt cannot be cast to aag
at baz.a(SourceFile:156) ~[1.16.5.jar:?]
at aqa.f(SourceFile:1654) ~[1.16.5.jar:?]
... 20 more