It happens especially at the initial turn of a game. If you save there and reload the saved game, the game messes up. It only occurs with TBC (using v61 now but happend earlier too)
I remember it happened during a longer game session too sometimes when (re)loading a game.
The only way to reproduce it now is to start a new game with TBC.
1. Let the Settler build the first city
2. Save the game
3. Load the same game again
Then you get like this here:

Buttons won't work anymore, unit icons vanish, and you are totally stuck then.
I tested this with cleared cache folder too and it still happens.
I remember it happened during a longer game session too sometimes when (re)loading a game.
The only way to reproduce it now is to start a new game with TBC.
1. Let the Settler build the first city
2. Save the game
3. Load the same game again
Then you get like this here:

Buttons won't work anymore, unit icons vanish, and you are totally stuck then.
I tested this with cleared cache folder too and it still happens.