Although we say that the window period for HIV testing can be up to 12 weeks, it is often much shorter. This depends on the type of test being done. The average window period for the 4th generation HIV test is 16-18 days, and we think over 95% of people infected with HIV will have a positive test 4 weeks after infection. So, to answer your question, in practice, we are not seeing people whose HIV tests are negative at 8-10 weeks after a risk then have a positive test at 12 weeks (unless they have another risk). It is still possible this could happen, but it is quite rare (less than 5% of the time). If you are concerned, you can always get a test at 12 weeks to feel absolutely sure that you did not get HIV from that encounter.
I hope this helps answer your question. If not, please submit another one.