Debugging Apache Bad Request

I got the following error this morning:

Bad Request

Your browser sent a request that this server could not understand.

Additionally, a 400 Bad Request error was encountered while trying to use an ErrorDocument to handle the request.

I didn't see anything in the Apache log file, so I increased the LogLevel from warn to debug: LogLevel debug. Turns out I was using an underscore (_) in my URL!

Apache log:
AH02415: [strict] Invalid host name '', problem near: _test.
AH00550: Client sent malformed Host header:

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