An observer placed so as to observe any block (any orientation) is triggered when powered or unpowered redstone dust adjacent to the second block is destroyed. (but not when the dust is placed) This was originally seen when observed block was an observer but seems to happen with any block, even air.
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May be a duplicate of MC-107760, not found because "odd circumstances" did not match my search keywords. 😉
In attached screenshot, observer on right triggered when sticky piston retracted, destroying redstone on the block it pulls