Typically, U.S. Presidential candidates enjoy an uptick in support or polling immediately following their party's nominating convention (even if it might be only temporary).
Obviously, conventions were contested more frequently in the not-too-distant past, and that can be a crapshoot in terms of uniting the party or attracting independents. But have there ever been uncontested conventions, that nevertheless hurt the nominee simply due to how poorly they went?
I have a short answer for you, but first, two caveats:
I think this would be very difficult (perhaps impossible) to measure from the era before modern polling.
Before and partially during the 1970s, political conventions served as the actual forum for the party to choose the candidate, rather than the sort of coronation/giant infomercial we are used to seeing today. This means that measuring the before and after convention numbers for individual candidates could produce a result with different meaning from a modern result, since it wasn't necessarily clear before the convention who would be the actual nominee.
OK, now to your question: the poll numbers for the democratic candidate (measured as two-party vote share) fell immediately following the convention in 1972. George McGovern was an ideological outsider to the party and lacked the support of many delegates at the convention. There was a floor fight for nominating the vice-presidential candidate with many rounds of voting, finally leading to McGovern and his vice presidential candidate giving their acceptance speeches very late into the night.
The democratic vice-presidential nominee in 1972, Thomas Eagleton, was the immediate subject of controversy when it was publicly revealed that he had undergone electroshock therapy and multiple hospitalisations for depression. A few days later, McGovern booted him (well, he resigned) and got a new vice presidential nominee. Since this all played out within about 2 weeks after the convention, it is perhaps difficult to say that the 2 point drop in the polls was exactly because of the convention. McGovern and the new choice, Kennedy-by-Marriage Sargent Shriver, lost overwhelmingly to Richard Nixon in the general election.
Political Scientist Thomas Holbrook wrote a book in 1996 called "Do Campaigns Matter?" which in part examines the history of convention bounces. Holbrook updated the figures from his book in a 2008 blog post, so you easily can see his figures for yourself. The graph he posted shows that, at least using the method he uses to measure the effect, every other convention since 1964 has produced at least a minor convention bounce.
Now I want to talk about what this all means in relation to this year, but I won't!