We have tested this from two different Windows PCs as clients (Windows 7 64 bit custom build PC and Windows 8.1 on Microsoft Surface 2 Pro) and only began to notice this problem with 15w42a, it did not appear to happen as recently as 15w41b. (alternately, one of the older snapshots apparently sent people to an unknown location not near a portal when going from overworld to nether? We're not seeing that anymore..)

What we are seeing now is that sometimes when you try to travel between nether and overworld, on arrival you can only see your destination (standing in destination portal) for a fraction of a second before you get sent right back to the source portal.

Other times you arrive in the destination portal, standing there as you should in a portal field with swirlies picking up, where you can either step out or just sit there like a nausiated fool if you please.

Most of the time this happened with one linked portal pair (we had another pair that never demonstrated the problem) where trying to go from Nether to Overworld bounced you back to Nether about 90% of the time. But one time my son did get caught in a loop between the two (starting from Overworld, bounced off of nether, bounced back off of overworld again, repeat) for at least 8 bounces before finally being able to step out on the Nether side.

In the server logs, these attempts are normally roughly paired with some combination of lines like:

[21:53:38] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running XXXXms behind, skipping YYY tick(s)

[21:53:40] [Server thread/WARN]: Tried to add entity Item with pending removal and duplicate UUID 6810395c-42a5-4ef1-b194-56c07b9e796b
(this one oft repeated 5-20 times)

[21:50:02] [Server thread/WARN]: PlayerNameRedacted moved too quickly! -108.14051233114319,5.0,-181.85948766885681

(worth noting: neither end of the portal linkage is as low as y=5! They are both in the 64-80 range..)