Is allowing takebacks in friendly games a bad practice? When I am playing friendly games with my brother, if one of us makes a big blunder that would make it pointless to continue playing, instead of quitting, we undo the move(s) to give a second chance (if we both agree, otherwise the one that did the blunder loses).

We do it frequently and the fun seems to increase since we can keep playing a balanced game.

But is this a good habit? Are there negative consequences of doing this?
Getting too used to playing with takebacks could be somewhat detrimental if you were to transition to tournament chess at some point, since it downplays the importance of keeping your guard up and being vigilant about tactical possibilities in positions. Nevertheless, I think that in the setting you indicate playing with occasional takebacks (especially for really major blunders) is an instructive way to go, and so can be a good short-term practice for your long-term understanding of the game.

When some terrible blunder happens in your friendly games, there are basically three choices:


Resign and start anew.
Play on in a very lopsided (and probably not so instructive) position.
Take the blunder back and pick a different way to proceed.


Only the third option forces you to return to the position where the mistake was made and figure out something better to do, which in turn requires you to determine the nature of the mistake that was made in the first place. This is a good thing to do, at least if you want to emphasize learning over the competitive results of your friendly games.