Of note, we played on a table with a tablecloth. This meant that the bottom pieces did not slide very much. If we had played on a slippery surface, it's likely that some bottom pieces might have slid a bit to the side during some of the placements. There's nothing in the rules that addresses this situation. I would imagine we would not consider a slide to be a fall, but does anyone have a house rule to cover sliding bottom pieces?
I think there is already a rule in the game that covers that possibility:
Remarque: Si la pose d'un bloc en déplace d'autres sans les faire tomber sur la table, le mur reste dans cette nouvelle position.
Note: If the bricks already in the wall shift position while a new brick is being placed, but do not fall down, the wall remains in the new position.
The idea is this makes it all the harder for the next player.