This is how to crash Notes 6.5.4 german (at least).
Create a blank database. Create a form with nothing on it, but a new line. Create a subform with one table on it. In that table create a hotspot button and switch its type to LotusScript (you don’t even have to enter any code, I have “insert Option Decare automatically” enabled, though). Below the button, create one field (default settings are OK).
Now, insert the subform in your form and off you go, watching NSD.
Seems to work without the table as well, but from my findings so far, without the table, the button has to be above the field, while this seems not to be the case if you put them inside the table.
Have fun restarting Notes. Can anybody confirm or test this with other versions?