British Guards Pass Out During King Charles' Birthday Parade Rehearsal

King Charles will celebrate his 75th birthday this year, which, if you're in the Royal Family, calls for a massive parade ... but it's the guards dressed in uniform that are paying the price for the bash.

Rehearsals were underway Saturday for Charles' Bday bash -- complete with a massive band who was forced to perform in sweltering temps of around 86°F ... about 20 degrees warmer than usual for this time of year across the pond.

It wasn't long before band members started dropping like flies, passing out from the heat ... and forcing medics to carry them off on stretchers like in a war zone.

At one point, a trombonist, dressed head to toe in a wool uniform went down, attempting to get back up and play before he was whisked away by medical reps.

Prince William, obviously aware to what had happened, tweeted out a thank you to all involved and noted the crazy temps for the area.

If you're familiar, you might know Charles' actual birthday isn't until November, but Palace officials decided it would be too cold to celebrate with a parade during that time of year.

Obviously, that decision makes Saturday's disastrous rehearsal that much more cringe ... but other than the few fainting episodes, everything else seemed to go off without issue.