Mashable wrote an article based on what was shared by Jenna Fischer and Angela Kinsey in episode 49 of their Office Ladies podcast:
12 watermelons were purchased for the trampoline test scene, and the first 10 or 11 takes didn't go well, so Daniels thought they'd
have to use a CGI watermelon in the end.
Miraculously, a watermelon finally bounced off the trampoline and onto the Chrysler 300 in the parking lot. The melon reportedly dented
the roof and broke the electric moon roof, which cost the production
team $6,500 to fix.
From the transcript of the episode (the "Kent" they're referring to at the start is Kent Zbornak, a producer on The Office):
JENNA FISCHER [00:46:58] Yes. I also had a question about the
ANGELA KINSEY [00:47:03] Oh OK.
JENNA FISCHER [00:47:04] And I was not alone. Lauren Brightwell,
Meghan Keall, Ciara Begley, Phoebe Bono and Maddie McKee all wanted to
know, "How many watermelons were used for the trampoline test scene"?
ANGELA KINSEY [00:47:18] How many?
JENNA FISCHER [00:47:19] Kent told me that we purchased 12
watermelons. But around take 10, Kent said to Harold Ramis, I think
we're going to need to CGI the watermelon thing because none of the
watermelons that he was throwing off the roof were coming anywhere
close to hitting the car.
ANGELA KINSEY [00:47:35] Yeah. They talk about it in the commentary
that they, they weren't getting it until take number 11.
JENNA FISCHER [00:47:42] Oh, Kent said it was the last watermelon that
it was take 12.
ANGELA KINSEY [00:47:46] Oh, Harold-.
JENNA FISCHER [00:47:47] Did they say it was?
ANGELA KINSEY [00:47:47] Harold Ramis said it was take number 11.
JENNA FISCHER [00:47:51] Oh well. So guys, it was either take 11 or
12. I guess by some miracle it landed on the trampoline and it hit the car perfectly.
ANGELA KINSEY [00:48:00] And I feel like Steve and Rainn's expression
like is real because they were like, "Oh, my god, we finally did it".
They return to discussing the scene a little while later:
JENNA FISCHER [00:50:49] So fun. Well, I have some more news about the
scene before we move on.
ANGELA KINSEY [00:50:53] OK.
JENNA FISCHER [00:50:54] The car that they hit was a rental. It was a
Chrysler 300 picture car. Guess how much money it costs them to do
ANGELA KINSEY [00:51:04] Oh, my gosh. I don't know, how much?
JENNA FISCHER [00:51:07] They dented the roof and they broke the
electric moon roof of the car.
ANGELA KINSEY [00:51:12] From that one watermelon?
JENNA FISCHER [00:51:14] Yeah. It cost 6500 dollars to fix it.
Note that the timings in the transcript don't seem to correspond with the podcast as presented online. I assume that advertisements etc. have padded the episode. The actual time where this section starts is approximately 50:30.