During sample preparation and analysis, samples are coming in contact with different labware materials. By four unrelated analytical (phytochemical and pharmaceutical) case-studies and employing different analytical techniques, we demonstrated the potential misinterpretation of analytical results due to the use of contaminants-leaching labware during sample handling. Oleamide, a common polymer lubricant and a bioactive compound, was identified as a main analytical interference, leaching from different labware items into solvents, recognised as chemically compatible with the tested polymer material. Moreover, anti-inflammatory effect of oleamide at 100 μg mL-1 and considerable pro-inflammatory effect of the plastic syringe extractables (containing oleamide) at the same level were shown in a TLR4-based bioassay. Taking these results into account, together with the fact that oleamide can be a compound of natural origin, we would like to notify the professional public regarding the possible erroneous oleamide-related analytical and bioassay results due to the use of oleamide-leaching labware. Researchers are alerted to double check the real source of oleamide (labware or natural extract), which will prevent further reporting of false results. Analysis of procedural blanks with de-novo developed UHPLC-ESI-MS method is, among some other strategies, proposed for detection of oleamide interference and avoidance of misleading results of certain analyses.