Interslice: Well yeah that is the easiest fix. But if OP wanted to have multiple drawers open he may have to do something like that. Cheers

Thanks . Sometime ago I did install as small kitchenette  in a house, that uses the same blum tandombox fittings,  and the runners on those were all predrilled at the factory. That didn't have this issue, although the gaps between the drawers were perhaps a few mm's larger. I don't think the integrated handles make any difference, because you would still have the top of the drawer that it would hit. The side adjustments behind the side panels appears to only move the front panel relative to the runners, eg up down or sideways, or for releasing them, and it does not appear to adjust the  runners themselves. It appears that that has to be done on the runner itself. I have emailed Blum about it with photos, so I can get an authoritative statement from the manufacturer to back to the kitchen guy with just incase . I can see him saying that people don't have multiple drawers open at the same time. But I disagree, as I often do that to move stuff between drawers, and I haven't encountered the issue before with any kitchens I have used, and it doesn't occur on all drawer units.

Dealing with this sort of frustrating issue is why I prefer to do things myself, rather than having to keep getting tradespeople back to correct things. When doing stuff myself, I can spend more time making sure it is done properly.  I have just installed some wardrobe drawers that use a similar a drawer system to the Blum one, which Cabaks produce, and it all works perfectly. For anyon wanting a wardrobe and have some DIY skill, they seem to be a good company to use.  That is also helped by the pre drilling of the holes for the drawers at the factory. All I have to do is make sure they are square and parallel with the other runners.