Yes, seen diesel instead of kerosine ordered often by people wrongly or delivered wrongly by fuel company. Not a big deal if standard eff. boiler with conventional vertical flue. Usually smaller nozzle with higher pressure needed. But any of the well baffled std. eff. boilers tend to not work well with diesel, - Grant Euroflame or Multipass are examples.
HE oil boilers cannot be run on diesel, although if there was a small amount of diesel in a tank of kerosine it won't do any harm & diesel is a lubricant on the oil pump.
On the point of mixing diesel with kerosine, Agas need PURE kerosine & not from a tank on a delivery lorry that has been used for diesel, OR if the previous delivery was diesel & the hose is still full of it! Some lorry men have the hose pumped dry into the lorry so to fraud customers, as when the lorry starts filling your tank it registers the litres but is actually filling the dry hose first!