Response for one compound in one sample run from batch way too high - otherwise OK.


I have a bizarre issue that has occurred once before. One of my in-house quality controls is around 280% too high ( see figure and sample ID K0900-11). The sample is injected twice during the run and both injections result in similar results. Here is what I have found out so far: 

Information on run: 
Matrix : Blood
Gradient run on 1,7µm 2.1x100 mm BEH C18 column
Mobile phase A: 0.1% Formic acid in MilliQ water
Mobile phase B: Acetonitrile 
Injection volume: 5 µL 
  1. Issue is not internal standard. We are using D4-Clonazepam and the IS area is similar in these samples to other samples. 
  2. Issue is not mixing of the QC samples. As you can see, the sample is labeled K0900-11. So this is vial 11 from a batch. K0900-10 which was run in the previous week, and K0900-12 which was run in the week after are both within 20% of expected values. 
  3. This is not due to too much sample in the vial. All other compounds in the QC sample are within 20% deviation from expected values. Only Clonazepam is so high. 
  4. This is not the injection volume since the IS response is similar to other injections and also the IS should correct for a difference in injection. 

Does anyone have a suggestion what the issue could be? As I say, this has happened once before and seems to be very compound specific and the internal standard is not correcting for the issue.