Steps to reproduce. Reproducible every time.
Open Flash CS6, draw a big (classic,input) text field, convert it to a library symbol, then remove the symbol from the stage.
Go to the library, double click the symbol you just created in the library to edit it, then double click the text field to edit the text (currently blank).
Press "Enter" key ten times to make ten blank lines, and then press the backspace key ten times to delete the lines. Flash crashes upon the tenth keypress. If by chance it doesn't crash right away, just press enter a few more times times and backspace an equal number of times and it will crash.
I took a video of it happening at least 3 times. I was editing a project file, trying to make a field just big enough for ten lines by creating the blank lines and dragging the text field handle to resize it, but it was crashing when I tried to remove the blank lines.
Since I was able to make it happen on a brand new file drawn from scratch as well, I figured it was a serious bug and not just a problem with the file I was working with. Also, it seems to occur only when the textfield's type is "input". It stopped happening if I changed the type to "dynamic".