If you are in spectator mode (/gamemode <3|spectator> [Name/Selector]) and you sprint while on or in a block, you see particles.
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What I expected: Particles would be invisible.
What Happened: Particles are visible.
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REPRODUCTION:
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1) Go into Spectator Mode ('/gamemode 3')
2) Go into/onto a block
3) Hold CTRL/your Sprint key
4) See your particles
Best seen if you are being supported on a block (i.e. not flying) and then switching
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Possible duplications:
MC-94535 - Flying and holding CTRL really close to the ground, emits walking particles
(doesn't mention spectator mode/must be on a block)