Alternate scenario, in which the worst thing happened at every turn.

1) In 2018, client took some amount from their 401(k), and reported this as a taxable transaction on their 2018 return, and the math worked out however and now they have a big chunk of cash under a mattress.

2) In 2019, the client took their $94,000 wad of cash down to Fidelity, and said "put this in my nondeductible IRA for me," and Fidelity assumed had the client sign a paper claiming that it was a timely indirect rollover, and put it into an account where nondeductible IRA contributions had previously been made, which at the time had a basis of $100,000 and a FMV of $200,000. Total FMV is now $294,000.

3) In July 2020, the account had grown from $294,000 to $300,000. Client requests that the $6,000 growth be distributed. Custodian withholds 20% / $1,200 and sends the remaining $4,800 to the client, who goes and buys summer holiday gifts with it.

4) In July 2020, the client has $94,000 distributed to him and stuffs it under his mattress, leaving $200,000 in the IRA.

5) In December 2020, client contributes $94,000 to a Roth IRA.

Tax consequences of that mess:
2) Client has an excess contribution of $94,000 for 2019, which is subject to a 6% excise tax for each year until it comes back out.

3 & 4) I think this comes out excess contributions first, and it would take me a couple of hours to track down the code section references and manually calculate the earnings, but my eyeball guess is that all the excess would be out (with no tax because it's excess), and all of the earnings would be out (taxable + potential penalty if under 59.5), and there would be an additional amount which would run through the 8606 to determine taxability.

5) Another excess contribution.