Data transmission problems at month-end
Back in the early 80s we had a problem reported from one of or satellite offices in France. A girl's terminal connected to a mainframe in the UK was suffering repeated retransmits. The mainframe would send a screenfull of data followed by a checksum. Her terminal would verify the checksum and if it didn't match would request a retransmit. She said it was happening several times an hour. After a couple of days she stopped reporting it so we didn't pursue it.
But then the week after another girl in the office reported the same issue. She was more vocal so myself (programmer) and a comms engineer planned to fly out there on the following Monday with a datascope so we could view the traffic and try to work out what was going wrong.
When we got there on the Monday morning she sheepishly told us that the problem had gone away. Just in case, we hooked up the datascope on her line and waited. Nothing happened.
On the Tuesday a third girl in the office started reporting the problem. We attached the 'scope to her line and sure enough the screen data was frequently arriving corrupted.
One of the girls then red faced announced that she knew what was causing the problem. She had notice that the problem affected any girl in the office who was menstruating. Anti-static mats under all their chairs solved it.