When the agent forgets the key (or their keycard or something).  Here, sometimes after it's gone to contract, the lock box is removed. One time I had to wait an hour for the agent to go back to office and return.

Have also been greeted by the police an an expensive, but vacant home due to alarms.

Showing before 9 AM and the lock box restriction prohibits the electronic box to open before 9 AM on the dot.

Going on a scavenger hunt to find the key. 

Showed a property in a rural area to a couple with some concerns for safety. Nobody disclosed the alarm system. It went off and the potential buyers were impressed. As we huddled out front to get the code, the county sheriff's patrol drove by and waved but did not stop! NEXT!

Agents not responding to requests for showing.

Restricted times. I had a seller who wanted me to list her house, but buyers could only view it for 2 hours on Saturdays. I declined. It sat on the market for nearly a year. 

Beyond that, tenant occupied properties. I had one last year where the tenants demanded they be present. They would walk about a foot behind the potential buyers. I begged the seller to let them go before listing, but she didn't want to have a month where she had to pay the mortgage. I eventually dropped the listing. She gave up about $8000 in profits because of the tenants.

A few things that have happened re: showings:

Bees nest in front door area near lockbox.

Broken key in lockbox.

Can't find lockbox (listing agent when called: "oh, I don't know, I didn't put it on")

Very limited hours for showings.

Seller followed buyers around telling them all the things wrong with the house.

Seller standing outside staring in window with arms crossed and scowl on face.

Rejected request for showing with no explanation and no response to phone call.

Cat/dog issues (instructions: don't let the dog out, actual: there was a cat, not dog, it escaped)

Tenant occupied properties are the worst.

I had a listing for both sale and rent that I managed. It was tenant occupied, MTM and showings were always a problem so owner and I decided I would attend all showings. Tenant was given 24 hours notice of showing and assured me that she and children would take dog for a walk.

I arrived early to find tenant, boyfriend, children and dog all sitting in dining room while dinner cooked. Buyer's agent arrived to show and they did not move an inch...dog barked incessantly, tenant followed agent and clients around with a running narrative and it was a disaster so lost a potential buyer. Owner instructed me to give tenant notice (month to month lease) and no more showings until tenant moved out.

That is just one example of many I could give but you get the idea...

'Listing agent must be present for showings' and commandeers the showing.
'Key in LO' and given to you on a ring with 50 other keys, not all marked.

'Call LO for appt' and they don't answer calls, texts etc.

'Call SELLER for appt' and seller causes you to REALLY rethink whether you want to proceed.

LB won't pop. 
Directions lead to nowhere.
'Alarm code xyxy333918637863336333' which invariably trips alarm, now we got cops and noise that won't stop and boob sweat. LOL

Tenants have always been the worst obstacle.

One of these factors could be an issue but the real challenge seems to begin with a combination of two or more plus the lockbox or key problem as previously mentioned by  Debbie Gartner, Kat Palmiotti AND Margaret Rome Baltimore 410-530-2400.   

1. Listing agent requiring financials of buyer.

2nd. Buyer agent not reading agent remarks. (showing instructions)
3rd.  No shoes.

There was the time I called an owner to get approval to show a house and was told, "No problem. No one is home." We entered and it was soon evident that someone was in the hall bath running the shower.

Quick exit! We did not go back.

The wrong key in the lockbox and the agent insisting it worked when they used it or the right lockbox, but the wrong combination.  Is it operator error?  Maybe, I have had that happen too.  Ha!

Actually at these prices and risks there shouldn't be a "list" of why we cannot do business as intended? Excuses do not create wealth but instead feed the ego and sustain the pride. Results don't lie and lack of them doesn't either

Black panther on the screened porch...a live one.

Aligator pit in the front room...with 6 ft gator...

Husband and girlfriend in the bedroom having sex (wife at RE office)...

Naked man sprawled on bed pleasuring himself...

Red house...all red carpet, red walls, only red clothes in closet with 4 female tenants with long black hair and red clothes...

Dirt hole in living room for devil worshipping...my buyer actually went to contract on it and then lost his deposit when the wife refused to close on it.

I swear none is exaggerated...

Eve

No key in the box.

The alarm was not turned off ... my entry set it off ... did not have code ... while calling my client who was at work ... the police showed up.  

Unbelievably we still have some agents that want you to pick up a key. Not at their office but at another that is only open 1 day a week between 10 & 2.

The missing key.

The owner reprogrammed the alarm and shortened the time to enter the code before the alarm went off. I couldn't get to the panel and enter the code in time before the alarm went off. The alarm company would call him then he would call me.

Tenant who won't cooperate.

Showing instructions say go and show.  No mention of silent alarm.  Surprised why the cops showed up???  Really???  Of course my get out of jail card was my real estate card!

Fred,

From commercial real estate days...we had a beautiful little architectural gem office building (3000 s.ft) with kitchen with a Japanese garden listed, and when I went to show it, there was a homeless person who had made it his home.  He even had a Christmas tree.  Luckily, I was showing it to a professional athete, so I felt safe, and the owner who had his office next door proceeded to fix the problem... A

No key in the lock box. an agent who HAS to be there but has no time to be there. DOGS and 3 day notice

tenants

The list is long but the one I will never forget was the gentleman in the shower and buyer screaming when she walked in. The home was listed as "vacant" and the gentleman in the shower was the listing agent.