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Just before Starship SN5's 5 August 2020 flight, there appears to be a leak:

white plume on left side of rocket

Was this planned or accidental?

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Do you mean the trail of white vapor coming from the lower side as shown in the image below? enter image description here

That is pre planned and you can actually see something similar when a Falcon 9 (or most other rockets) is preparing for a launch.

As I understand it, its from the LOX (liquid oxygen) boiling off as it warms up inside the tanks. The gas is pulled from the tanks through bleeder valves to help reduce stress from excess pressure (as I understand). Since the raptor engine also uses liquid methane, which has a temperature similar to that of the LOX, gaseous methane may also be expelled, although that is highly unlikely due to the risk of explosions from the mix of methane and oxygen and the contamination caused by unburned methane. You can see the cloud because the gas is still quite cold and water in the air condenses.

note- the image shown is from a screenshot of the official Spacex recording of the launch.

edit- changed part regarding methane.

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    $\begingroup$ Yup, you always see such plumes with rockets carrying cryogenic propellants. $\endgroup$
    – Loren Pechtel
    Aug 6, 2020 at 0:24
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    $\begingroup$ Is it oxygen boil-off or is it the helium that is bubbled through the cryogenic fuels to keep them cold in the tank? $\endgroup$
    – Moo
    Aug 6, 2020 at 1:16
  • $\begingroup$ Are you sure liquid methane leak is not dangerous? Isn't there a risk of explosion? $\endgroup$
    – Joe Jobs
    Aug 6, 2020 at 10:44
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    $\begingroup$ The methane tank is at the top, so this is clearly oxygen. I can't remember whether it is also vented into the atmosphere, or always vented into the flare stack which is slightly out of the frame in this still. $\endgroup$
    – Jörg W Mittag
    Aug 6, 2020 at 12:28
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    $\begingroup$ @LonelyFox Yes, using helium for pressurization is very common, and yes, with the exception of SRBs, rockets do use liquid propellants. But the "bubbling" that Moo suggested refers to a special pre-launch subcooling procedure used on the Falcons. $\endgroup$
    – TooTea
    Aug 7, 2020 at 6:19

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