I file 83(b) election, but did't include a copy of it in that year’s tax return
I've talked to several very experienced accountants that deal with startup shares, stock 83(b)'s, etc. weekly (based in SF, CA) as this issue would have had a massive impact on me.  The most important part of filing an 83(b) is notifying the IRS within 30 days. The law requires the written notification within the 30 day window. Adding it to that years tax return is an IRS procedure. Forgetting to include a copy of that years tax return is apparently a common occurrence when no tax was owed (0 spread, you actually paid the FMV). And the accepted method to resolve this is to simply file a blank amendment for that years return and include the copy of the 83(b) election.