ILLUSTRATION: GARY HOVLAND FOR THE WALL STREET JOURNAL

Generous donors give their money, their time and sometimes their valuable connections to charities. Some also give unsolicited advice, demand favors and leave a charity wondering if their donation was worth the hassle.

Of course, charities are loath to complain directly to any donors, especially big ones. And most donors have no idea when they are crossing the line.

With that in mind, here are some of the mistakes donors make, at least in the eyes of those on the receiving end of their largess—and how donors can avoid the offending behavior.

THE MISTAKE: GIVING UNWANTED GIFTS

The situation: Some donors have something they want to give away, even if that something isn’t what the charity wants. Joshua Lamont, a communications expert whose career has focused on philanthropy and the nonprofit sector, says a customer once told him about a donor who insisted a charity accept his large taxidermy collection of animals killed on safari. The stuffed animals had nothing to do with the charity’s mission of arts education, but the donor wouldn’t take no for an answer.

The staff was sent on a “wild goose chase” to find a way to resell the items, says Mr. Lamont, director of corporate communications atBlackbaud Inc., a provider of software to nonprofits.

The fix: Realize that an intended recipient may fear giving offense by saying “no, thanks.” If a donor wants to give away, say, a rare stamp or safari collection, Mr. Lamont recommends making sure the charity can use the items to fulfill its mission so it can receive the “related use” tax deduction.

THE MISTAKE: GIVING THE WRONG GIFTS

The situation: Even if a charity normally would take certain kinds of gifts, there can be a mismatch during times of emergency. When devastating floods hit Colorado in 2013, a lot of donors brought clothing and household supplies to the offices of Mile High United Way, says its president Christine Benero. But what the group needed was cash and other resources to provide housing for victims, Ms. Benero says.

While their generosity was greatly appreciated, it took time to explain to donors that what they brought wasn’t needed for the flood-recovery effort and to recommend where those items would do the most good.

The fix: “Doing a little homework in advance always helps provide donors more information on how to fulfill a particular need,” Ms. Benero says. Check a charity’s website to find out what’s most needed, or call the charity before dropping off unsolicited goods.

THE MISTAKE: ASKING FOR A LOT FOR A LITTLE

The situation: Donors deserve to know how their funding is making a difference, but they sometimes get carried away, says Melissa Berman, president of Rockefeller Philanthropy Advisors in New York. Ms. Berman recalls a donor who gave $2,000 to a charity and wanted a 20-page custom report on the donation’s impact in return. “It probably cost the nonprofit more than $2,000 in time to produce the report,” she says.

The fix: When donors want information on results, they should start by asking the nonprofit how it tracks and report on results, Ms. Berman says. “If you really need customized information, at least be sure the nonprofit agrees that your approach is feasible.”

THE MISTAKE: MICROMANAGING

The situation: Donors who micromanage the projects that their gifts fund can be a real headache for charities, says Ajay Kaisth, a financial planner in Princeton Junction, N.J.

Mr. Kaisth recalls a donor who gave a gift for an exhibition of women’s art. While the event was still being planned, she constantly asked for updates about the art coming in, sought permission to choose which art to show, and wanted to know every detail about the opening—down to the drink menu.

The fix: While donor involvement is appreciated, micromanagement is not. “Don’t put constraints on the charity’s ability to produce an event that best meets its needs,” Mr. Kaisth says.

THE MISTAKE: EXPECTING PERSONAL FAVORS

The situation: Sometimes donors think their gifts entitle them to unusual things in return. Laura Fredricks, a fundraising expert in New York, says she knows of an elderly man who was thinking of making a bequest to a charity and who subsequently asked the charity’s development officer to mow his lawn, drive him to doctor’s appointments and do some work on his car.

“The donor preyed on the development officer’s vulnerability, implying that if you do this…I will give,” Ms. Fredricks says.

The fix: “Donors need to realize the development officer isn’t there as a personal assistant or hired help,” she says.

THE MISTAKE: MAKING FALSE PROMISES

The situation: Donors who make a verbal pledge but don’t follow through can put a charity in a difficult position, says Betsy Brill, a philanthropy consultant in Chicago.

Ms. Brill describes how a high-net-worth donor who had supported a certain charity in the past told the executive director at a fundraiser that she was “very excited” about a new initiative and pledged several million dollars. The executive director counted on that amount for the following year’s budget. But when he followed up with a formal proposal, the donor had changed her mind. The charity was left in a precarious position and was unsure if the program could move forward.

The fix: “Donors shouldn’t make promises they can’t keep,” says Ms. Brill, and “nonprofits should never book a pledge until it is signed, sealed and delivered.” Ms. Brill recommends enlisting the help of prestigious fellow supporters to seek commitments in writing.

THE MISTAKE: RESTRICTING GIFTS

The situation: A restrictive gift can unnecessarily complicate things for the charity. For instance, a domestic-violence center in Ann Arbor, Mich., received a small gift that was restricted to helping pets of women who stayed at the shelter, says Robert Oliver, a financial planner on the shelter’s finance committee. Pets rarely factored into the shelter’s work, but the bookkeeper had to account for the gift separately, and the money just sat in the bank until it could be used for its stated purpose.

If the donor had understood the charity better, she would have trusted it with unrestricted funds, Mr. Oliver says.

The fix: “Research the charity so you understand what they need and give an unrestricted gift,” he says.

THE MISTAKE: BEING A KNOW-IT-ALL

The situation: Donors often confuse providing money with having wisdom, says Sammy Politziner, co-founder of Arbor Brothers, a New York-based philanthropic organization.

When a successful businessman recently met the executive director of one of the charities Mr. Politziner works with, the potential donor asked a few general questions, then launched into a monologue about how he would approach developing the organization.

“He didn’t ask [specific] questions first, didn’t listen and wasn’t asked for advice,” Mr. Politziner says. While the donor was just trying to help, he hijacked the agenda and didn’t know what he was talking about. “Unsolicited, peanut-gallery suggestions about what’s best for participants or how a program should be run are particularly galling to nonprofit leaders,” Mr. Politziner says.

The fix: Mr. Politziner says, no matter how experienced, donors should first ask: “What challenges are you facing that you think I might be able to help with?”

Ms. Dagher is a reporter for The Wall Street Journal in New York. She can be reached atveronica.dagher@wsj.com.