I have been writing about the games from the U14 county team events for many years now (12) and often end up saying the same things. This year we had a couple of particularly clear examples of "the most important mistake in chess" ... which is:

ignoring your opponent's threats!

Here's a simple example: Black has just played 14...Nb4 and White replied with 15.Bh6

Black went ahead with their plan to gobble the c-pawn:

15...Nxc2?? 16.Qxg7#

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Here's a not much more complicated example: 13.Ne5?? Qxg2#

In both cases, the players who lost were more interested in what they were doing than their opponent's threats.

By the way, you may see from this how pointless it is to move a piece, and hold onto it while checking for problems. If you are holding onto it, you can hardly see the board anyway, but... the problem might not be with the piece in your hand, it's the piece you have undefended!