> With BSD-licensed code, that risk is way smaller. The worst you have to do is add a few (or quite a few thousand. Go check Setting/General/About/Legal for an example) attribution lines to the versions of your code that use the BSD-licensed code.
Accidentally copying GPL-licensed code into non-GPL-licensed code puts you in exactly the same legal position as accidentally copying BSD-licensed code without attribution: you are now, technically, violating copyright, and in theory liable for statutory damages of $150,000. You're never forced to release your code under GPL; the GPL copyright holder might insist that you either release your code under GPL or pay the $150,000, but equally the BSD copyright holder might insist that you pay the $150,000 - they're within their rights to do that.
In practice the author will most likely be willing to work with you to sort it out in either case.
> Worse, the source code of all versions from _before_ that checkin automatically become GPL-licensed (because they contain code that, transitively, becomes GPL because it later was linked with GPL-licensed software)
This is simply false.