Definitions of unreliable:

• adjective:   lacking a sense of responsibility
• adjective:   not worthy of reliance or trust

  Example: "In the early 1950s computers were large and expensive and unreliable"

• adjective:   dangerously unstable and unpredictable

  Example: "An unreliable trestle"

• adjective:   not to be trusted
• adjective:   liable to be erroneous or misleading