This has sparked a memory
Around the same time as the events described I worked for a big Pharma company. The company was developing one of the first monoclonal antibody-based anti-cancer therapies, and my role was to devise a test to measure biological activity for early-phase material. We had a nice little assay going using Chromium 51 (medium-hard gamma emmitter) in fresh lymphocytes, and in order to validate the test a second team in another building were also required to reproduce my results.
All went well at first until one day, a couple of weeks in, they lost sensitivity and started having bizarre high backgrounds and unpredictable results. This came and went over the next couple of weeks, with things getting worse after a period of more intense work, then dropping back to normal levels. The more they tried to fix it, the worse it became, then would suddenly work properly again.
The gamma counters we used at the time, made by LKB Wallac, were pretty much industry standard and usually used for counting Iodine 125, a softer gamma emmitter and were legendary for reliability. The basic design had the gamma detectors in 2 rows of 6 with lead shielding between to reduce crosstalk, samples were placed in a rack by hand which was then loaded into the instrument for counting over a fixed interval.
After this had been going on for a couple of weeks I wandered down to talk things through, see if there was anything 'obvious' in their technique that might cause trouble. On reaching the lab, the cause became immediately apparent - the waste bag containing 'hot' waste from previous experiements was being stored under the bench, directly beneath the counter. When the team had previously used Iodine, the gamma from that had been too weak and used at too low a level to penetrate the lead shielding, but the Chromium was being used in much larger amounts, and the higher energy gamma would go straight through a centimeter or so of lead shielding.
We moved the bag, the assay performed like normal again. The treatment eventually made it to clinical use. Lab coat of course. :-)