What verb means "to make something erroneous or wrong"?
The context is
However, in the first trial of the experiment, the yeast was siphoned through the device into the other test tube, _____ing the results.
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The results have been contaminated.
Contaminate verb Make (something) impure by exposure to or addition of a poisonous or polluting substance - ODO
This definition relates to the substance being literally contaminated by the contaminant, but the effect of the contaminant on the results can also be described with the same word. Here's an example:
... some outside astronomers said the group had underestimated the extent to which interstellar dust could have contaminated the results ... - Dennis Overbye
Returning to your sample sentence, we have:
However, in the first trial of the experiment, the yeast was siphoned through the device into the other test tube, contaminating the results.
If something was deliberately made wrong, with malicious intent, consider sabotage.
any underhand interference with production, work, etc., in a plant, factory, etc., as by enemy agents during wartime or by employees during a trade dispute.
If it was perhaps not necessarily malicious, but interfered with in order to produce a deceptive result, then you might use falsify.
1. to make false or incorrect, especially so as to deceive: to falsify income-tax reports. 2. to alter fraudulently.
Examples from Dictionary.com