By Eyk Henning in Frankfurt, Max Colchester and David Enrich in London 

One of the main goals of Europe's yearlong banking stress tests was to provide the public with reliable, comprehensive data about the finances of the continent's lenders. But some errors and inconsistencies nonetheless crept into the test results, which were published Sunday.

The European Central Bank had to briefly remove from its website the results of a large Italian bank after discovering an error in its key capital ratio. Results of a review of Polish banks' balance sheets were left out of the tests due to the late submission of data. And the ECB and the European Banking Authority, which were jointly overseeing the testing process, came up with drastically different figures for an important Deutsche Bank AG data point.

The issues appear to be isolated and, unlike in previous iterations of the European stress tests, don't call into question the overall credibility of the exercise, according to analysts and other experts. They said some mistakes are inevitable when compiling over a million pieces of data.

The stress tests, and an accompanying review of the quality of banks' assets, assessed the ability of 150 leading European banks to withstand a deteriorating economic and financial environment. All but 13 banks were found to currently have ample capital, a result that officials said reflects the industry's generally strong financial footing.

Investors nonetheless dumped shares of European banks, including those that easily passed the tests. Bank stocks in the Stoxx Europe 600 index tumbled more than 2% Monday as some investors worried about whether the tests will do enough to restore confidence in the industry.

The biggest selloff was Italy's Banca Monte dei Paschi di Siena SpA, whose shares plunged nearly 22% after the bank was found to face a EUR2.1 billion ($2.7 billion) capital shortfall.

The Tuscan bank hired outside investment banks to advise it on its strategic options, but analysts and bankers said those choices are likely limited due to the bank's financial problems. Italy's markets regulator imposed short selling bans on the shares of Monte dei Paschi and another lender that failed the tests, Banca Carige SpA.

Other hard-hit banks included Lloyds Banking Group PLC, whose shares fell about 2%. Lloyds narrowly passed the European tests but now faces a tougher exam being conducted by the Bank of England.

While the tests have been going on since last year, European officials were scrambling until the last minute to finalize the data. On Sunday morning, barely an hour before the results were due to be released, the EBA official in charge of the stress-test process was still scrambling to rubber stamp a final version of the report, causing him to show up several minutes late to a briefing with journalists.

Shortly after the results were published, ECB officials detected an error in the 2013 capital ratio of Monte dei Paschi, Italy's third-largest bank, according to a person familiar with the matter. The error, on the first page of a template posted on the ECB website, was important because Monte dei Paschi was the worst performer in the stress tests and therefore was at the center of investor attention Sunday.

After discovering the error, ECB officials briefly removed Monte dei Paschi's files from its website, according to the person familiar with the matter, who described the problem as an isolated "data-processing error." The corrected files were republished a short time later. But their temporary removal drew the attention of investors, some of whom privately expressed frustration that the ECB had altered the figures without explaining what had changed.

A Monte dei Paschi spokesman declined to comment.

Other banks had data issues too.

While the stress tests were conducted using banks' Dec. 31, 2013, data as a starting point, banks were told to provide regulators with data for the first nine months of 2014 about expenses they've incurred as the result of litigation and regulatory investigations. That item is a nod to the mounting toll inflicted on banks' finances by escalating penalties imposed by regulators in the U.S. and elsewhere.

Deutsche Bank, facing a wide range of lawsuits and government investigations, has been a focal point of such concerns, and so its litigation-expense figure was being closely watched.

The EBA pegged Deutsche Bank's litigation costs for the nine months of 2014 at EUR470 million, which was the figure Deutsche Bank reported for the first six months of the year. The ECB's figure was nearly triple that, at about EUR1.4 billion.

The reasons for the discrepancy, which didn't affect Deutsche Bank's capital ratio, aren't clear.

An ECB spokeswoman said it used data covering Deutsche Bank's litigation expenses for the first nine months of the year, adding the data was provided by banks to the national supervisors. An EBA spokeswoman said the agency used data that was provided to it by the ECB. A Deutsche Bank spokesman declined to comment.

Many other banks simply didn't disclose their litigation expenses at all. The ECB spokeswoman declined to comment on why some banks didn't provide the data, adding it asked banks to provide fines and litigation costs incurred in the first three quarters and net of previous provisions.

Meanwhile, late-arriving data affected the results of the six Polish banks that were subject to EBA tests. A footnote to the EBA's report on the tests says the results for Polish banks don't include the impact of an asset-quality review because of a "late submission" of data by Polish regulators.

A spokesman for the Polish Financial Supervision Authority said the regulator did conduct the requisite asset review, but that the results were delayed due to certain unspecified "verifications" with Polish banks on Saturday. That review knocked about two percentage points off one Polish bank's capital ratio, meaning it would have only barely passed the stress tests.

Write to Eyk Henning at eyk.henning@wsj.com, Max Colchester at max.colchester@wsj.com and David Enrich at david.enrich@wsj.com

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