According to recent announcements from Pokemon Pocket, there appears to be some developmental concerns with the upcoming Mega Evolutions set. This could cause some unexpected delays for affected parties.
The Announcement

TPCi (The Pokemon Company International) has just announced on the Pokemon TCG UK X account that they'd be moving back the release schedule of the Mega Evolution expansion for the Pokemon Trading Card Game (TCG) for certain markets.
Originally scheduled for a worldwide release on the 26th of September, TPCi has specifically mentioned that their EMEA (Europe, Middle-East, and Australia) markets will be suffering this setback. For players in the affected markets, you can expect to see a release on October 10th.
The only reason TPCi had given for the delay is logistical issues. What this means is anyone’s guess, but that does bring up a question for the markets that will experience an unaffected release schedule.
Tournament Legality

While the rest of the global market remains unaffected by this delay, some members of the online community have been inquiring whether the delay will impact the tournament legality of cards from the Mega Evolutions set. Unfortunately, there has been no word yet from TPCi on whether any upcoming regional tournaments will be affected at this time.
As best we can speculate, if TPCi decides to keep all the regions on equal footing until October 10th, then the major regional tournaments being held in Milwaukee, WI and Belo Horizonte, Brazil might be affected. These two particular regional tournaments will be held almost at the same time of the delayed release schedule.
For players planning to attend the aforementioned regionals, it may be prudent to keep an eye out for any news that may come of this delay. The potential inclusion or exclusion of a new card set at the last minute could spell major modifications to existing deck lists and could affect the outcome of match-ups across the board.
This may be especially true considering that Mega Evolution Pokemon may pose a serious threat to certain deck types due to their bulkier bodies and dangerous damage outputs.