Yes!

If only one single input out of those 300+ does not sign the transaction, it is not valid according to Bicoin consensus rules, and other nodes will reject the transaction.

This can happen when a user shuts down his computer, or the wifi goes off, or a Tor node is overloaded, or the user is malicious.

The first attempt of signing a coinjoin transaction has only ~25% success rate.

Then the coordinator kicks out the non-signing input, and attempts a second round, which has ~50% likely hood of success.

So it usually doesn't take more than 5 attempts to succeed.