In Firefox, I have two tabs corresponding to Debian bugs.
One has a correct favicon, the other one doesn't.
See attached screenshot.

Both bug pages have

  <link rel="icon" href="/favicon.png">

So I don't understand why I get a difference, in particular
because in Firefox, with Tools → Page Info, then Media tab,
I get for https://bugs.debian.org/favicon.png

  The image “https://bugs.debian.org/cgi-bin/pkgreport.cgi?pkg=favicon.png;dist=unstable” cannot be displayed because it contains errors.

With wget:

qaa:~> wget https://bugs.debian.org/favicon.png                       <17:52:05
--2026-01-30 17:52:05--  https://bugs.debian.org/favicon.png
Resolving bugs.debian.org (bugs.debian.org)... 2607:f8f0:614:1::1274:81, 209.87.16.81
Connecting to bugs.debian.org (bugs.debian.org)|2607:f8f0:614:1::1274:81|:443... connected.
HTTP request sent, awaiting response... 302 Found
Location: https://bugs.debian.org/cgi-bin/pkgreport.cgi?pkg=favicon.png;dist=unstable [following]
--2026-01-30 17:52:06--  https://bugs.debian.org/cgi-bin/pkgreport.cgi?pkg=favicon.png;dist=unstable
Reusing existing connection to [bugs.debian.org]:443.
HTTP request sent, awaiting response... 200 OK
Length: 10406 (10K) [text/html]
Saving to: ‘favicon.png’

favicon.png         100%[===================>]  10.16K  --.-KB/s    in 0s

2026-01-30 17:52:06 (186 MB/s) - ‘favicon.png’ saved [10406/10406]

qaa:~> file favicon.png
favicon.png: HTML document, Unicode text, UTF-8 text, with very long lines (511)

which explains the error.

Then after a forced reload (Ctrl-Shift-R), I no longer get the
correct favicon.

Note that https://www.debian.org/Bugs/ does not have such a
<link rel="icon" ...>, and this page has the correct favicon.