Especially because, wouldn't you know it, both understudies had fallen ill.
But never fear - help was (literally) just around the corner.
Mamma Mia's general manager had remembered that one of the show's previous stars - Steph Parry - was currently working as an understudy in 42nd Street, which was playing just down the road.
He called Steph, to ask whether she could step in, and within just 18 minutes of Mamma Mia! being halted, she was on the stage playing Donna - the role made famous in the film of the same name by Meryl Streep.
As a stand-in for two of 42nd Street's leading roles, she was sitting in the dressing room at the Theatre Royal on Drury Lane when she got the call.
The theatre is a hop and a skip from the Novello Theatre, the home of Mamma Mia!
"The general manager told the audience: 'I'm really sorry, unfortunately Caroline has injured herself and is unable to carry on in the show and we have no understudy - however, we do have a lovely lady who will be carrying on for her shortly,'" Steph explains.
When the understudies took charge
Welsh actor Owain Arthur took over from James Corden as the gluttonous minder Francis Henshall in One Man: Two Guvnors in 2012
David Tennant's stand-in Edward Bennett received a standing ovation when he stood in for him as Hamlet in 2008. As well as being the stand in, Bennett had been playing the role of Laertes
Natasha J Barnes had to step into Sheridan Smith's shoes for several weeks in Funny Girl at the Savoy Theatre
But how on earth did she remember it all - the steps, the blocking (where to move) and the songs? (Although to be fair, we could probably all manage the latter - this is Abba we're talking about).
"Last time I played the role I was on a cruise ship and it was the US version of Mamma Mia - the blocking and choreography are different, even the script is slightly different," says Steph.
"I didn't have time to worry, I just needed a wee! I remember sitting on the loo thinking about the lines for the next scene. After that I had no need to think, I just had to trust that it was somewhere in the back of mind. It was worse, the more I thought about!"
She says the dancing was the biggest challenge.
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"The choreography was the biggest wing in the whole of my life - I blagged it!" she says, adding: "The cast were great, I knew they were on my side."
And there were other challenges - she only knew one or two of the actors.
"Jacqueline Braun, who plays Rosie, I'd never met before. She came up to me just before I went on and said: 'I just have to say hello, we're about to play best friends of 20 years, nice to meet you!'"
So how has she managed to carve out a career as the go-to understudy?
"My whole career has been [like that] - on my second night in Wicked I had to go on as Madam Morrible. My costume wasn't ready and I didn't have a wig!
"Ten years ago I was in the Pyjama Game in the Union Theatre - the lead role Babe lost her voice on the afternoon of the opening night, they had asked me to understudy the role but not officially.
"So I had to go on with a script! The next day, the director said I had to learn my lines for that night because 'Time Out were in'."
And the jobs kept coming.
"You get known as someone who is reliable. I'd love to be a leading role, that's where I'd like to be. But what's great is you do get the moment to save the day. Moments like that are so special."
And last week's performance was a particular career highlight.
"It was like nothing I've ever experienced in my life. At the end of Winner Takes It All, I felt like Celine Dion!"
But Steph says she is genuinely surprised by all the media attention.
"I didn't think it was anything special, I just did what I did. The publicity has blown me away."