It's important to remember that an Extended Test is really a series of tests performed over time, and any one of those individual tests could be critical to the success of the overall endeavor.
p.58 of the SR4 rulebook:
The gamemaster can also limit the number of rolls under the
assumption that if the character can't finish it with a certain amount
of effort, she simply doesn't have the skills to complete it. A good
limit is to allow a maximum number of rolls equal to the character's
dice pool (so a character rolling 6 dice has 6 attempts to get it
done). A character can also fail an Extended Text by glitching (see
Per your question, you can also fail by glitching. See p.59:
If the character rolls a glitch during an Extended Test, the task is
not aborted, but the character should be inconvenienced in some way
(broken tools, missing parts, or distracted from work by an annoying
pest, for example). The gamemaster can also roll 1D6 and subtract this
from the hits scored so far; if this reduces the accumulated hits to
0, the test fails.