I've done some testing since this happened in a live game and it does not happen reliably but it does happen often. The bug is compound in that I believe there is a visual component to it as well.
Have a board with trashing (I used Upgrade, Butcher, and Catapult)
Play the Inheritance event onto Fortress.
The Crash Bug:
Trashing the Estate/Fortresses will eventually lock up the client and no amount of logging out, refreshing, etc. will unlock it.
The crash occurs when you select a card to trash and press the TRASH button to execute the trashing. The TRASH button goes away but the "You may trash a card" text remains above the player's hand and at the bottom of the log.
The Visual Bug:
The Estates do correctly come back to hand and in cases such as Upgrade or Apprentice the on-trash-benefit is correct for a 2 cost card being trashed. However, when the Estate comes back to hand it appears visually to be a standard estate and no longer bears the modified test of Fortress (although it will continue to behave as modified Estate until the crash).
I have waited the game out for awhile and then hit refresh and the visual bug was corrected in hand but the interface was still locked up. However the log read:
plays a Butcher
trashes an Estate
puts an Estate
into their hand.
Estate loses track of Estate (it moved).