I answer questions and they are placed sometimes first and sometimes last.does it mean something?

It depends on the settings you have dictated.

Where it says SHOW, on the right above the first answer box, you can change it to read -

All answers/Oldest to Newest/Newest to oldest/Rated highest.

It just depends in which order you want to see the answers.

I have answered questions, pressed submit, and "oops, something wrong".

Answer has vanished altogether. Really annoying.

Do you mean first as you been chosen? Or do you mean where your answer appears on the page itself?

Sometimes, I pick question, so far zero answers, submit and find 3 other answers submitted at same time.

Suppose you are the first to answer a question, your name will come first.

If you are the last to answer, then your name will come last.

That's all!