He broke his expectation. أخلف رويعاً مظنه

أخلف رويعاً مظنه
أصله أن راعياً كان اعتاد مكاناً يرعاه فجاءه يوماً وقد حال عما عهده. أي أتاه الخلف من حيث كان لا يأتيه. ومظن كل شيء حيث يظن به ذلك الشيء. يضرب في الحاجة يعوق دونها عائق.
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He broke his expectation.
The origin of this saying is that a shepherd used to tend his flock in a certain place, but one day he came to it and found it had changed from what he was accustomed to. That is, something unexpected came to him from where he did not usually come. The place where something is expected is where it is believed to be. This proverb is used to describe a need that is hindered by an obstacle.

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 المثل العربي «أخلف رويعًا مَظِنَّه» يُضرب لمن خاب ظنه فيما كان يعتمد عليه أو يتوقعه، أو لمن وجد العقبة حيث كان ينتظر التيسير. وأصله أن راعيًا قصد المرعى الذي اعتاده، فوجده قد تغيّر وخلا مما كان يرجوه.

أي: أتاه الإخفاق من الموضع الذي كان يظن أنه مصدر النجاح أو المنفعة.

وأقرب المقابلات في الثقافة الإنجليزية هي:

• Things didn't turn out as expected.
  • لم تسر الأمور كما كان متوقعًا.
  • يوافق المعنى العام.
• The best-laid plans of mice and men often go awry.  
  • خير الخطط كثيرًا ما تؤول إلى الفشل.
  • مثل إنجليزي مشهور يُضرب عندما تُفسد الظروف ما كان يُظن أنه مضمون.
• Expect the unexpected.
  • توقع ما لا يُتوقع.
  • قريب من جهة أن الأمور قد تخالف الظنون.
• There's many a slip 'twixt the cup and the lip.
  • بين الكأس والشفاه زلات كثيرة.
  • أي قد يعترض النجاحَ عائقٌ في آخر لحظة، وهو قريب جدًا من قولهم: «يضرب في الحاجة يعوق دونها عائق».

أفضل مقابل

إذا كان التركيز على اعتراض عائقٍ دون بلوغ المقصود، فأفضل مقابل هو:

There's many a slip 'twixt the cup and the lip.

أما إذا كان التركيز على إخفاق التوقعات والخطط، فأفضل مقابل هو:

The best-laid plans of mice and men often go awry.

وكلاهما أقرب إلى معنى المثل العربي من أي ترجمة حرفية.

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The Arabic proverb "He failed to deliver what he expected" is used to describe someone whose hopes were dashed by something they relied on or anticipated, or someone who encountered an obstacle where they expected ease. Its origin lies in the story of a shepherd who went to his usual pasture only to find it changed and devoid of what he had hoped for.
In other words, failure came from the very place he thought would be the source of success or benefit.
The closest equivalents in English are:
Things didn't turn out as expected.
Things didn't go as expected.
This agrees with the general meaning.
The best-laid plans of mice and men often go awry.
This is a well-known English proverb used when circumstances spoil what was thought to be guaranteed.
Expect the unexpected.
Expect the unforeseen.
This is similar in that things may not go as expected.
There's many a slip 'twixt the cup and the lip.
There are many slips between the cup and the lip. That is, success may be hindered at the last moment, which is very similar to the saying, "He is in need, but an obstacle prevents him from achieving it."
Best equivalent:
If the focus is on encountering an obstacle before reaching the goal, the best equivalent is:
There's many a slip 'twixt the cup and the lip.
But if the focus is on the failure of expectations and plans, the best equivalent is:
The best-laid plans of mice and men often go awry.
Both are closer to the meaning of the Arabic proverb than any literal translation.