Missed the subsequent tax window on 31 of 90 certificates because I ran one calendar
Portfolio was $290k across 90 certificates, nine counties in four states. The plan was geographic spread. What I actually built was nine separate procedural calendars and I tried to run them off one spreadsheet tab with a column called "sub tax due."
The windows for paying subsequent-year taxes on a certificate don't line up. Different months, different rules about whether paying is optional or required to hold priority, and in most of these counties nobody mails you a reminder because they have no duty to. I missed 31 of them.
Two consequences. In one state a later-year certificate on four of my parcels went to another buyer at the next sale, and by that state's statute that buyer sits ahead of me on the redemption waterfall. I ended up buying two of those out at a number I would not have paid on purpose. In another state my failure to keep subsequent taxes current reset my clock to start the foreclosure process, which pushed two positions out by about two years.
Cost, as best I can pin it: roughly $18k of interest I never earned, $6.2k in legal to figure out where I actually stood in two of the states, and about $4k of overpayment buying out the competing certificates. Call it $28k against a portfolio that earned $34k that year.
What I'd do differently: build the per-county calendar before I register to bid, not after I hold paper, and cap the number of jurisdictions at what one person can genuinely track. Nine was vanity.