This question actually arises from board games like Risk and Axis & Allies. In those games, it is very easy to forget about troops you have in one area (planning to move them during your turn but just forgetting about it) and that forgetfulness leads to them getting taken out. Has something like this ever actually occurred in war?
Perhaps not “forgot about them” but does “lost track of them and unaware of their continued existence” as for Sparrow Force meet your criteria?
On Timor during WWII from 23Feb1942 (on the surrender of most of 2/40th Battalion after fierce fighting) higher command believed all of Sparrow Force lost, while 2/2nd Independent Company combined with remaining elements of 2/40th Battalion to continue dispersed raiding and ambushes against the Japanese with the strong support of the Timorese people. Higher command was unaware of their existence until they were able to cobble together a radio from parts and contact Darwin on 20Apr1942. Resupply was established in late May1942.
The military units mentioned are Australian but the combined force included some Dutch Troops.