In football, transfer deals simply can't be considered safe or foregone until a player's signature finds itself on the requisite line below a club contract, no matter how many agreements are made. Certain clubs have learned all too well just how embarrassing it can be when they officially announce deals with players only for them to sign for other teams.
This transfer window has produced two notable examples so far and there could be a few more to follow, given the competition for top players in the market nowadays.
Premier League sides, especially, don't have much time left as their window shuts before the start of the season this year due to a new agreement. So you'd do well to expect a huge rush approaching the final day of the window.
With regards to deals getting hijacked, it's something that happens all the time and it's become part of football. Over the years, there have been some shocking u-turns made by players whose clubs agree over deals, as well as players who shake hands over terms themselves.
Unfortunately, there's no surefire way for clubs to safeguard against such occurring from time to time. But, on the flipside, these embarrassments don't come around too often.