Thanks DutchNikon,
I checked all that and I still get about one degree of the picture leaning to the left. So it may well be the viewfinder which I don't understand since the camera was not subject to any shocks. May be something got loose ?…
I will send the D300 for re-alignment. It's getting old and needs a bit of servicing any way. It may cost a bit of money but it's a lovely camera giving very good results.
…And the D400 is not going to appear on the shelves for a long time :)

Did you also test this on a tripod, using a remote control ?

A lot of ppl. move their camera slightly when depressing the release button.
This happens often because they "sqeuze" when releasing...

When its the camera still showing this effect, then the sensor my need realligning, or the viewfinder itself got misaligned. This can be checked by putting the camera on a tripod exactly horizontally, and using "LiveView", and viewfinder for the same picture . when Liveview is OK, then the sensor is OK ...

Hi everyone !
Just noticed recently that all my pictures are leaning to the left about 1° even though I make sure I keep the horizon straight. Has anyone had the same problem and is it a costly repair ?
My D300 was not dropped or anything. I cannot imagine what the cause may be…

Thanks !