One of the interesting failed COVID vaccine efforts was by the University Of Queensland in Australia which was working on a novel protein based vaccine where the idea was a conformationally locked COVID spike protein was injected[1] - basically it presents the protein as it's found on the surface of the virus, which in turn promotes an antibody response which is "accurate" - whereas free-floating proteins, i.e. if you just shredded up the virus - don't look the same.
The problem? The technology was based on a protein sequence called gp41 - which is a subunit of the HIV spike protein. It's not HIV, it's not derived from actual HIV virus - it's made in labs from separately cloned sequences...but unfortunately, part of the immune response to the vaccine generated HIV binding antibodies, similar enough to "real" HIV binding antibodies that they would trip false-positives in HIV tests - i.e. you would test positive to HIV for months, but you didn't actually have HIV - you had antibodies which had enough activity to the common HIV test assay that it looked like you did (e.g. a PCR test for HIV would show you don't have it. But it's impractical to have a whole lot of people who look indistinguishable to the HIV positive population when we had other vaccine options).