Not a 'one off', but for a short time pacers were allocated to Cornish branches. Very occasionally they reach Penzance (I'm pretty sure I read that on this forum, no less).
There was a 'Visit Bristol' liveried 143 operated by Wessex Trains that somehow made it all the way to Penzance on one occasion. This particular 143 carried a little St. Piran's flag badge on its cab afterwards, in celebration of its achievement!
More recently, Exeter based 143s have occasionally made it to Penzance in the absence of other stock being available or line blockades. It's happened about once or twice a year since late 2015.
Increasingly 143s have been working the early evening 2P93 Exeter - Plymouth and 2E46 return in recent weeks, owing to the shrinking numbers of 153s available (which normally do the run).
Excluding normal passenger workings, of course the the 'Farewell 142 railtour' by FGW did a couple of jaunts over unfamiliar tracks around Devon, including the Heathfield Branch in 2011.
When new, the 142s did operate many of the Cornwall and Devon branch lines; in place of heritage DMUs. But the flange noise and wheel wear was deemed unacceptable. They also greatly struggled in leaf fall season - their little shopping trolley wheels could not cope with the steeper gradients and tight curves / track cambers in greasy conditions. Pacers remain banned on the Cornish branch lines, but are cleared for the SW mainline down to Penzance.