Our 'Masked Singer' sources tell us, no contestant has ever threatened to quit so close to the start of the season before ... so everyone was caught off guard, and without a backup plan ... they were pretty much gonna be SOL.

Basically, execs of the show would have to completely revamp the entire season ... just before it was set to began. Luckily though, they were able to do some serious damage control ... and the show went on as planned.

We've learned there are a number of things that played a part in the contestant's initial decision, ranging from feeling ill-prepared to feeling overly restrained in the costume/mask.

We know what you're thinking ... but this wasn't a case of stage fright. We're told this particular contestant is no stranger to live performances on stage, but the giant costume completely threw them off.

These kinda "wardrobe malfunctions" aren't new to the folks at 'Masked.' As we previously reported, the complexity of the costumes has raised lots of concerns ... ya know like onstage wipeouts, safety of the contestants etc.

In this case, crisis was averted ... that is, after MS producers sent flowers to the performers and gave some much needed pep talks and praise. Turns out, the contestant ended up being a really strong performer and we're told they have the potential to win it all.