Are firms sophisticated maximizers, or do they appear to make mistakes? Using transactions data from Ugandan value-added tax returns, we show that sellers and buyers report different amounts 79% of the time, despite invoices being easily cross-checked. Our estimates suggest that most firms are “advantageous misreporters“, but that 25% are “disadvantageous misreporters” who systematically overreport own sales minus purchases such that their tax liability increases. Similarly, many firms—especially disadvantageous misreporters—fail to VAT-report imported inputs they themselves reported at Customs, increasing their liability. On net, unilateral VAT misreporting cost Uganda about US$384 million in foregone 2013-2016 tax revenue.