What are the consequences of not including a conte

The RFC says the content-length header is optional ("..Applications SHOULD use this field...").

From what I can gather if its not included then the client will not know how much data to expect, therefore will not be able to show a determinate progress bar when downloading the body (i.e. the top bar instead of the bottom).

Are there any other side effects or bugs that arise from omitting this header?

I think your implicit question is "How does a client detect the end of an HTTP message?". See RFC 7230 - HTTP/1.1 Message Syntax and Routing - Message Body Length:

The length of a message body is determined by one of the following (in order of precedence):

Any response to a HEAD request and any response with a 1xx (Informational), 204 (No Content), or 304 (Not Modified) status code is always terminated by the first empty line after the header fields, regardless of the header fields present in the message, and thus cannot contain a message body.

Any 2xx (Successful) response to a CONNECT request implies that the connection will become a tunnel immediately after the empty line that concludes the header fields. A client MUST ignore any Content-Length or Transfer-Encoding header fields received in such a message.

If a Transfer-Encoding header field is present and the chunked transfer coding (Section 4.1) is the final encoding, the message body length is determined by reading and decoding the chunked data until the transfer coding indicates the data is complete.

If a Transfer-Encoding header field is present in a response and the chunked transfer coding is not the final encoding, the message body length is determined by reading the connection until it is closed by the server. If a Transfer-Encoding header field is present in a request and the chunked transfer coding is not the final encoding, the message body length cannot be determined reliably; the server MUST respond with the 400 (Bad Request) status code and then close the connection.

If a message is received with both a Transfer-Encoding and a Content-Length header field, the Transfer-Encoding overrides the Content-Length. Such a message might indicate an attempt to perform request smuggling (Section 9.5) or response splitting (Section 9.4) and ought to be handled as an error. A sender MUST remove the received Content-Length field prior to forwarding such a message downstream.

If a message is received without Transfer-Encoding and with either multiple Content-Length header fields having differing field-values or a single Content-Length header field having an invalid value, then the message framing is invalid and the recipient MUST treat it as an unrecoverable error. If this is a request message, the server MUST respond with a 400 (Bad Request) status code and then close the connection. If this is a response message received by a proxy, the proxy MUST close the connection to the server, discard the received response, and send a 502 (Bad Gateway) response to the client. If this is a response message received by a user agent, the user agent MUST close the connection to the server and discard the received response.

If a valid Content-Length header field is present without Transfer-Encoding, its decimal value defines the expected message body length in octets. If the sender closes the connection or the recipient times out before the indicated number of octets are received, the recipient MUST consider the message to be incomplete and close the connection.

If this is a request message and none of the above are true, then the message body length is zero (no message body is present).

Otherwise, this is a response message without a declared message body length, so the message body length is determined by the number of octets received prior to the server closing the connection.

When the server omits the content-length header, it has to use one of the other mechanisms to indicate the end of the message.

So to answer your question: scenarios 3 (chunking) and 7 (reading until the server closes the connection) are the ones where the client doesn't know the length on beforehand.