I used QGIS Raster->Extraction->Contour tool to extract contours at 1m interval from a virtual raster. The virtual raster was built using around 100 tiles in ASC format that represented a 1m DEM.
When zoomed out the contours looked as I expected conforming to the shape of the terrain very well, even picking up minor detail such as dish drains on the side of roads.
However when I zoomed in I noticed that there was numerous missing contours. For example, in the image below showing a ridge, the contours step down at 1m increments from about 101m (bottom of image below) to 96m. However then there is a significant jump to the next contour - some examples are 72m in the top right of the image, to 85m on the left of the image. Looking at the attribute table there also appears to be gaps in data (for instance if I filter display to show contours in the map, there are gaps)
I checked the virtual raster for this area and it looked fine.
I seek advice on how to obtain all the contours as the extracted contours are probably not useful in this state.
Below is a copy of the raster with the contours overlaid. Apologies for the lack of graduation, but you can see different shades of gray showing the terrain. Due to the size of the raster I had alot of trouble changing the styling.
Below is a copy of the settings I used to extract the contours also.