The bug
The exception thrown by the method net.minecraft.network.Varint21LengthFieldPrepender.encode(ChannelHandlerContext, ByteBuf, ByteBuf) (20w07a Mojang name) when it tries to encode a buffer whose length cannot be stored using 3 bytes is created with an incomplete message.
Example message
unable to fit 2232077 into 3
"unable to fit " + i + " into " + 3
Suggested would for example be
"Cannot write buffer length of " + i + " using " + 3 + " bytes"