Mainland visitors are proving the truth of the saying that it is possible to have too much of a good thing. More than 28 million came last year, outnumbering tourists from the rest of the world put together. Now, at the request of Chief Executive Leung Chun-ying, mainland authorities have agreed to put on hold plans to allow 4.1 million non-permanent Shenzhen residents to come here on multiple-entry permits.
Ironically, Hong Kong asked the mainland to send us more tourists during the Asian financial crisis to help revitalise our economy, and Beijing was happy to oblige. In fact, Chinese leaders have made it clear they want to help. So far, Beijing has never said "no" to any request from Hong Kong.
It was perhaps in the spirit of helping Leung that the central government allowed him to call in the Japanese consul general, Yuji Kumamaru, after a group of Hong Kong residents was detained by Japan after landing on a disputed island in the East China Sea.
It was natural for Leung to voice concern about Hong Kong residents and to call for their release. After all, it is his responsibility to ensure their safety.
Two years ago, then chief executive Donald Tsang Yam-kuen tried but failed to get in touch with the Philippine president, Benigno Aquino, while Hong Kong tourists were being held hostage in Manila. Tsang was trying to bring about the safe release of the hostages.
Under the concept of "one country, two systems", Beijing is responsible for defence and foreign affairs. However, Article 13 of the Basic Law says the central government authorises Hong Kong "to conduct relevant external affairs on its own". Presumably, the Diaoyu Islands issue was an instance when Beijing authorised Hong Kong to do so.
But it was odd that the chief executive should summon a foreign diplomat not just to make known his concern for the welfare of Hong Kong residents but also to protest over the sovereignty issue. That, after all, would seem to be a diplomatic matter, reserved for the central government.
Hopefully, Beijing would not fudge the line between the two systems simply for the expedience of helping to raise a chief executive's popularity rating.
Where Hong Kong's autonomy begins and ends is set out in the Basic Law, but sometimes Beijing's actions raise questions as to just where the lines are drawn. For example, Article 136 of the Basic Law says that the formulation of educational policies is within Hong Kong's autonomy. And yet, President Hu Jintao, in a speech to mark the 10th anniversary of the handover, said Hong Kong "should put more emphasis on national education for young people".
A few months later, Tsang cited this presidential advice in his policy address and announced that Hong Kong would "attach great importance to promoting national education" so students will "grow to love our motherland and Hong Kong", launching the disastrous policy.
None of these events - summoning the Japanese consul general, calling the Philippine president, proposing national education - was in itself necessarily a violation of the Basic Law.
But, given the importance of that constitutional document, great care should be taken by both Beijing and Hong Kong to ensure that the line between Hong Kong's autonomy and the central government's powers is always clearly and distinctly drawn.
Frank Ching is a Hong Kong-based writer and commentator. email@example.com  Follow him on Twitter: @FrankChing1