Tanks outrun the infantry. Or they continue the attack alone after dropping off their tank-riding grunts, without picking them back up again.

Assault without adequate artillery prep.
Recon with the expensive unit that's available instead of the cheap one that's two turns away.
Advance platoons piecemeal because they don't all have enough commands to change their movement objectives. In fact, just generally wasting commands is rather foolish.
Attacking with my Rangers/Partisans before they all arrive. Six Partisan squads on turn 9 are a lot more effective than 3 on turn 6, 2 on turn 7 and so forth.
Of course a lot of scenarios are specifically designed to force you to hurry. (Isn't that right WB?) But its crucial to strike the proper balance between your inner Patton and Montgomery.
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Target, Cease Fire !