How about investigators using light-boxes, X-rays or even endoscopes to see inside sealed envelopes? They've also been known to break into the trial co-ordinator's office late at night to find the randomisation list.
The sometimes extraordinary lengths investigators will go to subvert randomisation have been documented (Schulz KF) and evidence of bias in the randomisation procedures of published trials have been uncovered (Peto R).
Centralised randomisation is recognised as the gold standard method for treatment allocation in clinical trials. For instance, the ICH E9 Statistical Principles for Clinical Trials guidelines state
In multicentre trials (see Glossary), the randomisation procedures should be organized centrally.
One of the reasons for this is that it is very hard, or impossible, for investigators to subvert the randomisation process when that process is controlled centrally (see Allocation concealment in randomised trials: defending against deciphering for more details).
By using our centralised randomisation services, both you and others that review your work, can be confident that randomisation is unbiased. An additional layer of protection is provided by the independence of the randomisation database, held by Sealed Envelope, from other patient records held at the trial data-collection centre.