Lynn Newby, 40, was almost at verge of calling it quits with her boyfriend, Andy Allott, 29, after discovering that one of the Favourites was a packet of 12 condoms.
Newby accused Allott of buying them and having an affair behind her back.
She even threatened to cancel a holiday to Mexico because she refused to accept that he was innocent.
"I couldn't believe what I saw when I logged onto my Tesco account. It gives a list of everything bought on a club card and there at the top of the list was a packet of Mates condoms," Telegraph quoted her, as saying.
"I knew I hadn't bought them so I went in and accused Andy, who had not idea what I was talking about," she added.
When Tesco investigated the matter Newby was sent a letter of apology from Chief Executive Sir Terry Leahy - and a cheque for 100 pounds.
Sir Terry told Lynn that he could appreciate how distressing it had been for her and an investigation had revealed that the problem was caused by a new member of staff in the marketing department.
The new staff member had added the condoms as a "special promotion" to customers on line favourites but it had not shown up as a promotional item.
"This error was completely unacceptable," added Sir Terry and staff are being given further training " as a matter of urgency" to make sure it doesn't happen again."
Newby said: "Things are fine now and we can laugh about it but I just wonder how many other people had to deal with this mistake which could have cost me my four year long relationship."