Like two unwanted but unavoidable guests shaking up a harmonious party, immigration and drugs are two issues troubling otherwise improving relations between the United States and Mexico.
And since the two problems aren't about to go away, relations that have been on the upswing since Mexico first sought to join the North American Free Trade Agreement in the early 1990s can still anticipate rocky times ahead.
At the annual binational meeting between high-level Mexican and American officials held this week in Mexico City, Secretary of State Warren Christopher heralded a "new era of understanding" for a relationship historically marked by disregard and suspicion.
Some observers suggest the largest-ever US contingent at the 13th US-Mexico talks was an election-year parade orchestrated by the White House to put a positive spin on a potential campaign issue - Mexico - that could hurt the president.
Republicans, especially, have criticized the administration for orchestrating last year's $50 billion international bailout package following the peso crisis, and for not taking a stronger stand on what some consider Mexico's lax approach to fighting the drug trade.
But Mr. Christopher based his sunny assessment on what he called "frank and productive discussions" and a list of 11 agreements that ranged from border environmental projects to drug-fighting measures and an accord to enhance the protection of migrants and respect of their human rights.
Beyond this week's meeting, the US is also pleased by the historic recent extradition of two Mexicans to face charges in US criminal courts and signs that Mexico may be opening the door to increased military cooperation between the two countries to battle the drug-trafficking threat. Both extradition and military cooperation have long been touchy subjects for Mexico, which is sensitive to any suggestions that it is loosening its grip on its national sovereignty.
But underlying official accolades of improved bilateral relations is also recognition that the relationship faces some stiff challenges - and largely over two issues, immigration and drugs, that were downplayed by both countries especially as a reluctant US Congress mulled the merits of NAFTA.
The meeting's host, Mexican Foreign Relations Secretary Jos Angel Gurria, opened discussions by telling his American guests that Mexico is "deeply concerned by the emergence of trends that could jeopardize our relations and lead us down the road to confrontation."
Among the "trends" shaking Mexico are recent violent incidents affecting illegal Mexican migrants in California; accusations last month by Drug Enforcement Agency head Thomas Constantine that Mexican banks, especially on the border, are centers of drug-money laundering; and last week's passage by the US Senate of legislation, perceived here as anti-Mexican, that would double the size of the US Border Patrol while cracking down on alien smugglers and people who falsify immigration documents.
The problem of alien smugglers indicates how difficult it still is for the two countries to see sensitive issues eye to eye. US officials emphasize Mexico's agreement at the binational meeting to join in a crackdown on smugglers - "the real abusers of migrants' human rights," as one senior US administration official said.
Mexico does have a law against human "trafficking," and has recently stepped up its attacks on smuggling rings. But at the same time Mexican officials say privately that the smuggling business is only reinforced by the US when it doubles and triples the Border Patrol, since that forces migrants to seek help in crossing the border.
The "memorandum of understanding" on protection of migrants is another "agreement" that may in fact contain little meat. Mexican officials, including President Ernesto Zedillo Ponce de Len hailed the accord for "establishing the means for respecting the human rights of migrants," but US officials privately acknowledged that the agreement, sought by the Mexicans as a public-opinion pleaser in the wake of the California incidents involving Mexican migrants, contains nothing that isn't already part of consular agreements between the two countries.
"It is a useful way to make sure everyone is reading from the same sheet of music when it comes to the rights and treatment of immigrants," said one senior US administration official. US Immigration officials also emphasized that agreements reached earlier this year have stepped up communications between officials on both sides of the border.
Mexican officials also emphasized that some of the steps taken under the Zedillo administration that the US government is portraying as a warming-up to the US are actually part of a general push by Mr. Zedillo to forge a new, law-respectful Mexico.