To be precise they sent a letter to Spurs and copied in Arnesen, thereby circum navigating Spurs right to refuse permission, How careless!.
Interesting use of the word accident here, to most people it would be something akin to knocking over a drink.
In Chelsea's case this is stretching the case beyond credibility given that there was several Chelsea personnel involved & so it cannot be put down to one persons absent mindness.
For starters somebody (Private Secretary) had to type up the offer in draft form, and then the legal team would have to approve the offer as it is now a controlled document and finally a minimum of one (But usually two directors) would have to sign both documents.
(Frank Arnesens “copy” would also have had to be signed)
All in all a fairly lengthy procedure involving many people.
It is possible for two or more people to make a simultaneous mistake, these are known as contra errors.
The odds of an single contra error occurring are several million to one, given that there were more than 2 people involved, the actual odds of multiple contra errors are beyond normal computation and are also undocumented in the history of mankind.
Another possibility of course is that
Naturally I leave that choice to you dear reader.