Under what conditions could this occur?
At very roughly $120k-$130k (single earner; I forget the exact figure) you're no longer eligible for Roth IRA contributions. However, you can make non-deductible Traditional IRA contributions and then do a Traditional-to-Roth conversion (not "recharacterization"). This is known as a "Backdoor Roth IRA contribution" and is legal and explained in nauseating detail on the internet.
So:
> Under what conditions could this occur?
If you make more than ~$62k, have an employer 401(k), and contribute the maximum to it, funding a Roth IRA is a good idea.
(Unless you have an existing traditional IRA balance which would make backdoor Roth funding counter-productive due to the pro rata …. blah blah. If you don't have an existing Traditional IRA balance it's not a problem.)