This incident which happened yesterday evening seems quite odd: Yemeni jet mistakenly bombs army post, kills 30.
It is neither clear who really bombed those troops nor if it was indeed "mistakenly".
The attack happened near Zinjibar in the south of Yemen. There has been fighting around Zinjibar for several month and there is complex mix of groups and interests involved.

Since late May Zinjibar, a town with some 20,000 inhabitants, has been in the hand of rebels which are said to include militant Islamists and "al-Qaida in the Arab peninsula (AQAP)" fighters. Yemens military has fought bloody battles with those militants since then and several hundred fighters died on both sides. There is some suspicion that the take-over of the town was secretly supported by President Saleh to raise the specter of "al-Qaida" and to demonstrate to the U.S. that only his continued rule can deny its rise.
During June and July the barracks of the Yemeni 25th Mechanised Brigade near the city were surrounded and cut off by the rebels for over a month until the brigade was relieved through heavy tanks, aerial bombing, naval rocket strikes and tribal fighters who had earlier supported the rebels and are otherwise also anti-Saleh. Their tribal leader explained that there were now too many unwanted foreigners, mostly Saudis, with the Islamists and they did want these in their area.
In late July an air strike, allegedly by the Yemen air force, killed 15 to 25 of the tribesmen supporting the military offensive. The tribesmen claimed that they had given their own position coordinates to the government and accused it of intent. Another similar "friendly fire" incident happened shortly thereafter. Throughout June and July the U.S. was also involved in air attacks on the militants in Zinjibar with fixed wing aircraft as well as drones.
Another brigade in the area, the 119th Artillery Brigade defected to the opposition which is demanding for President Saleh to step down. The brigade did not join the Islamist rebels but just stepped aside. But in August the brigade was suddenly back on the side of the government and helped to fight the rebels. It is thought to have received significant support from the U.S. military.
The town was reported to be back in government hands on September 10 but that turned out to be premature as fighting has continued since then.
Today's bombing hit a position of soldiers of the 119th Brigade.
It is possible that this was an unintended "friendly fire" strike by the Yemeni air force. It is also possible that this was an intended strike as the 119th brigade is still thought to be anti-Saleh. It might have been a strike by U.S. planes which was probably given the coordinates to hit by Saleh's government. Saleh has used such a trick last year when he gave coordinates of a rival army commander's headquarter to the Saudis claiming they were for Houthi positions the Saudi troops were then fighting and bombing.
The story makes clear that the issues in Yemen are very, very complex.
But what is somewhat unexplainable in the whole Zinjibar story is that several brigades of the Yemeni military have not been able to free the town for over three month even with tribal help and U.S. support and against militants who have no obvious support or supply lines from outside. This makes the rumors of Saleh's secret support for these Islamists quite believable.
Saleh's strategy towards the U.S. is to present himself as the only man who can keep AQAP down and the country together. While the U.S. had earlier called for Salh to step down as president it recently allowed him to leave Saudi Arabia, where he had been for medical care after an assassination attempt, and to come back into the country.
It seems that Saleh's strategy has so far worked quite well for him and yesterday's air strike may have been just another of his tricks to keep the AQAP problem alive and himself at the top.