How to reproduce
Break a block (or try with a sword) in creative mode (survival mode is MC-46766),
Switch to spectator mode,
Click as if to break some block close by
→ You'll see 2-3 block-breaking particles
Note: Not sure if others in SMP can see these particles.
Code analysis
Code analysis by @unknown can be found in this comment.
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Confirmed for 14w21b

Confirmed in 1.8.

Confirmed for 1.8.1-pre2.

Confirmed in 1.8.3. Occurs when switching from any mode, not just creative. Also occurs if you try to break a block with a sword in creative mode.

I can also confirm this issue. Happens for a moment, and then stops.
EDIT: Nice catch, @unknown! Maybe we should create a new ticket for that, or change the summary of this one so it includes that.

Please link to this comment in the description
The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.
The reason for this is that the methods net.minecraft.client.Minecraft.sendClickBlockToController(boolean)
and net.minecraft.client.multiplayer.PlayerControllerMP.onPlayerDamageBlock(BlockPos, EnumFacing)
do not test if the player is in spectator mode.

Confirmed for 1.13.1.
Confirmed for 1.13.2 and 19w09a
Can confirm in 20w48a.
Can confirm in 21w03a.
Can confirm in 21w05b.
Can confirm in 21w06a.

Confirmed for 1.18-rc3


Whoops, sorry.