If the player is holding the sprint key and attacks something (like a mob or other player), this will not interrupt their sprint, as it does with a double-tap sprint. However, the server assumes that this does interrupt the sprint, and the player's server state is changed accordingly. This results in a situation where the client thinks the player is sprinting and the server thinks they are not.
One effect of this is that knockback distance is reduced, since that is calculated on the server. From the player's perspective, their first hit while sprinting will cause much more knockback than all following hits, until they restart their sprint. This is counter-intuitive, since the sprint could go on for any length of time, reducing knockback for no apparent reason, whereas restarting the sprint will instantly allow the extra knockback.
Here is a video demonstration of the bug:
https://www.youtube.com/watch?v=kp4h0O_YKYw#t=9m50s
Any fix for this will change gameplay noticeably. Preserving the client behavior and fixing the server to stay in sync will make sprint knockback much more powerful. Changing the client to match the server means that sprinting will be interrupted when attacking, which players may consider regressive.
Some possible compromises:
Fix the server to match the client, but reduce the sprint knockback bonus in general, to balance it out
Fix the server, and add a timed cooldown between sprint knockbacks
Fix the server, but use other factors to reduce knockback, such as the target's velocity and sprinting state
Linked issues
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Comments

Confirmed in 1.8. Also affects singleplayer.

Confirmed in 1.8.1 (also in SSP as above).

Also affects 1.8.2-pre1.

Is this still an issue in the current Minecraft Snapshot 15w47c or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
Yes, still an issue in 1.9.2
Confirmed for 1.13.1.

Confirmed for 1.14.3-pre1.
confirmed for 1.15 and 1.15.1

Confirmed in 1.16.1.
I personally think this should be fixed while also retaining the "feature" it brought, basically making it so only the first hit in a "sprint hit chain" deals the sprinting knockback while all the ones after that only deal default knockback until you reset your sprint (un- and then resprinting). The reason I am recommending this fix is that sprint-resetting in order to continuously deal more knockback is a wide-spread PvP combat mechanic.

I agree with Jonas. Since this bug has been reopened, I think the client-server desync should be fixed WITHOUT deleting the sprint resetting feature. Sprint resetting such as W-tap, s-tap, sprint-blockhitting are very hard and useful mechanic in both 1.8, 1.9 and CT pvp, so it should be kept. It adds a bit of skill, and is very intuitive.

Can confirm in 1.19.1.
I would like to request ownership of this ticket as the current reporter has been inactive since 2016.

Can confirm in 1.19.2.

Would like to add that this bug also affects how hunger is consumed: see https://www.youtube.com/watch?v=NiOMVtrkV-M
as well as how swimming works, see MC-220390. Probably related to MC-220390 as well.

This is an important component in PvP, as doing it allows you to do critical hits while sprinting. This should not be fixed.
@unknown, it's purely Mojang's decision on whether something is intentional or not. With this issue, critical hits while sprinting are only possible because of this bug, which is of course occurring due to a desync between the client and server. Besides, this ticket has already been assigned a Mojang Priority, therefore Mojang considers this to be a valid issue.

Jeb already asked about this during the development of the combat tests, ie: "should the sprint reset be kept for the combat tests?". And universally the answer was yes, because without this, sprint-knockback becomes overpowered and overcentralizing, and it removes crit chains.

I would like to reiterate that the fixing of this bug to fix MC-220390 would negatively effect PvP in a substantial way. The desync allows doing critical hits while sprinting by continuing to hold ctrl after connecting a sprint-knockback attack. This is a massive part of PvP and adds a lot of depth, removing the desync entirely would be a terrible change. The side effects of the desync, such as MC-220390, should really be the only things fixed. Alternatively, the ability to do critical hits while sprinting by continuing to hold ctrl after connecting a sprint-knockback attack should be coded in more intentionally. I cannot stress enough how important this is to PvP.

You do know that fixing a desync can fix it both ways, right? They can chose that the server side is correct, or that the client side is. Either way, it being desynced is straight up wrong.
I noticed this a while back and always complained about it. People told me that this is how it was supposed to me, but people would abuse it by constantly restarting their sprint to knock me back far. I made a video demonstrating it as well. https://www.youtube.com/watch?v=6sqF73mA33Y