Nothing gets your blood pumping then finding a critical software bug in your commercial accounting software while doing the taxes.
In this case some sales would be booked as credit instead of debit.
This seems to no longer happen with new entries but existing entries have not been automatically corrected (or the user informed about it), when this bug was fixed.
Just editing the respectig entries and saving them without making any actual change did fix them. Being a small business, this was „just“ about a hundred entries.
For the laymen:
This is something that should be absolutely impossible for any double entry accounting software. Both sides of the transaction no longer added up to zero. The money you take from A no longer matches the money you deposit in B and C.
We found this by chance because it happened often enough to result in one part of our income being reported as negative. While theoretically not impossible, at least this is extremely unusual and raises all flags.
Had we not found this, it would have resulted in our stated income to be off by serveral thousand Euro. When (not really an „if“ here) found in a tax audit , that would have been quite a legal affair. We double checked that no other year is affected and had to manually calculate the respective tax form entry to be on the safe side.