How to merge trees in a root file

[Wile_E_Coyote]

Aux;1 aux;2 aux3;

Hello! I’m filling a tree and after I, sometimes, get this funny behavior AUX;1 AUX;2 with the same histograms inside but with different events in each ;1 and ;2… Can somebody please tell me why this is happening? Many thanks, Stefanos