I originally suggested this as a mega event, but this is really a funny first encounter. This is based on the fact that in astrobiology theory have considered that other forms of life could evolve using other solvents than water, or have a different apnosphere. Someone other  than humans might be hospital like the A;tarians for instance  would offer u something to drink or eat assuming the toxic apnosphere might not kill u outright. The drink could be sulferic acid. ammonia, methane, carbon dioxide, oxygen, hydroclouric acid, clourine, or water. The food would be saturated with this stuff. If it was an acid it would eat some of u outright. They would notice that some kind of poison or disease replaced your blood with toxic water. They couldn't rick contamination so they would have to quarenteen both ambassadors while doing a confusing autopsy. By the time the Humans or this could work among 2 other races get a hold of the body they would suspect that he was poisoned. Is this an interstellor incident or is it war. This would peobably cause both species to be disrespectful to other species on the first encounter like refusing to accept the other soecies food or drink. They might assume this was a disease or a genetic anomally.

     There is another problem like when the europeans met the American indians or like the war of the worlds. Smallpox had decimated the western roman empire causing them to train the germanic tribes for warfare; because, they did not have enough people for an army. Eventually causing an end to the western roman empire. Eventually the body developed antibodies to smallpox and had become unconsequentual. Later on when the Indians had met the Europeans they had been exposed to a dangerous disease they hadn't experienced and had no immunities against it the rest is history. What would prevent this from happening to other species. This could cause some species to have gas masks, hasmat suits, or even space suits on first encounters with other species.

This is my ideas for a first encounter chance when u meet new species. I was wondering do u have any ideas for first encounter chance stories.