If UFO's truly are spacecraft from another world then one would have to realize that, like any built thing, accidents can happen. Even without knowing what makes them fly or who or how they operate, it is still a reasonable assumption that they can break or falter just as any machine can do. Undoubtedly the most famous supposed UFO crash was in the New Mexico desert near Roswell in 1947. This, however, was by no means a unique, or for that matter, rare event.

Still from video of KGB recovery of crashed flying saucer March of 1969 in the Sverdlovsk region of Russia.
When investigating UFO crash reports from around the world, the totals are surprising. Since the middle of the 20th Century with the advent of the "foo fighters" and "ghost rockets" seen across Europe during World War II until the end of the century, there were over 200 reports of UFO's crashing here and there around the globe. This average of 4 crash reports a year has not diminished with the onset of the 21st Century either. As example let us look at the brief time period between May of 2006 and January of 2007.
On May the 20th, 2006 a large number of eyewitnesses saw a flaming object crash into the sea near Port Shepstone, South Africa. The point of impact was just beyond the breaker line offshore from Port Shepstone High School. Along with school students and teachers who were outside attending a sporting event, local fishermen and other bystanders also saw the crash. Along with flames, the witnesses reported the water "exploding" and a cloud of smoke lingering for a short time over the spot the object hit. The National Sea Rescue Institute (NSRI) responded to the reports with police, rescue boats, and aircraft and covered a 12 square mile area but found no evidence of the downed object. Further checking showed no known aircraft either missing or overdue.
A mere seven months later the villagers between the towns of Yeniseisk and Lesosibirsk in Siberia reported a "flying apparatus" plunging from the sky at about 10 a.m. on the morning of December 1st, 2006. The crash caused a forest fire according to the Krasnoyarsk Territory Directorate for Internal Affairs. The Ministry for Emergencies stated that no aircraft were moving in the area at the time and that they had no reports of missing ail vessels. Investigators from the Transport Prosecutors Service and the Aircraft safety group Rosavianadzor rushed to the area.
Strangely, later that day Siberian officials released the statement that nothing unusual had occurred. Siberian Regional Centre of the Ministry for Emergencies spokesperson, Sergey Andriyanko, stated that all of the local witnesses "had reported false information" about the crash, and that "despite previous official claims to the contrary" there was now no evidence of the reported forest fire. "We report officially it has been a false alarm," Andriyanko said.
A single month later, on the 1st day of January 2007, South Africa was again the location of a UFO crash report. The Administrative Manager of the mining town of Lephalale, Leonie Ras, made the report. She stated that on that morning she was reading her email when she heard a noise like "an Airbus aircraft starting its engines and increased rapidly until the sound was like a million turbines screaming in unison." Running to a window she stated she first the clouds take on a bright orange-red color. Suddenly "a bright object plunged from the clouds to the earth, at a terrible speed, and hit the ground with an almighty bang." Her son-in-law, Cobus Nel also heard the sound but did not make it to a window in time to see the object as it fell to Earth.
The local police station was inundated with reports and questions about the bright light, noise, and crash but could provide few additional facts as their best guess was that the actual crash site was located in a heavily forested area just across the Border in Botswana approximately 28 miles away.
Less than two weeks later, on January 10th, 2007, another UFO crash was reported. Occurring about 60 miles from the provincial capital of Kerman, Iran, eye witnesses reported the object had been flaming and putting off a thick smoke before impacting the ground. People witnessed the falling of this fiery object from the sky from several surrounding villages. The Deputy Governor General of Kerman province Abulghassem Nasrollahi, stated that while the object made a large explosion on impact that no property was damaged due to the crash. Despite rumors that the crash was of a conventional aircraft, he reported that all such vehicles were safe and accounted for.
As with all UFO reports, there is a good chance that some of these many crashes were actually something mundane that had been misidentified. There are some, however, that defies all logical explanation. Regardless, the number of unidentified crashes does lead one to realize that even highly advanced technology has the potential to fail. One can also take in to account the possibility that sometimes the militaries of the world get lucky and shoot an unidentified aircraft down. Whatever else you may want to believe about UFO's, they are as subject to accidents as any other physical construct.