This reversible reaction occurs in room condition:
$$\ce{2NO2 <=> N2O4}\quad(\Delta H=-57.23\mbox{ kJ/mol)}$$
This is the Lewis structure of $\ce{NO2}$ (courtesy Ben Mills via Wikipedia):
This is the Lewis structure of $\ce{N2O4}$ (courtesy ChemSpider):
To me, it seems that no bonds need to be broken for the forward reaction ($\ce{2NO2 -> N2O4}$) to occur, which is strange indeed.